That's a big bird. Typically, for smoking, it's not recommended to go much over 14lbs or so, but that is with store-bought, injected birds. Well...let's see...unless your Cajun injector grates are something along the size of 28-30" wide and 20" or so deep, then, no, the bird won't fit laying on it's back. If you tied the bird up, then had a prop to set it on like for beer-can chickens, and your smoke chamber can accommodate the width of the bird, you could stand it up with the upper grates removed...but, that's one helluva lotta load on one grate, so you may have to stack 2 grates on each other to better support the weight and reduce grate sagging...possibly requiring that you fasten the grate frames together with mechanic's wire to keep them as strong as they can be without making more complicated and possibly permanent mods.
Spatch-cocked turkey would definitely not fit, however, quartering the bird may be a good option...cooking time would be about 1/2 that of a whole bird.
Just so you know, regarding the danger-zone temp/time rule normally recommended for birds (40-140*/4hrs), that is only recommended to follow with birds that are injected, such as most brands you find in the grocery stores or big-box outlets. Your bird was probably not injected with broth or marinade...you could ask the persona who butchered it just to be certain. That said, if it took 6 or 8 hours to get over 140*F internal temp, no harm done. BTW, I would recommend that you do not inject it with marinade, just to avoid the danger-zone temp/time rule.
If it was finished on grain, it's a bulky bird, fattened-up...probably not so much it's age making it so heavy...not like a roasting hen (chicken) as those are truly a tough old bird. Roasters do benefit from low & slow cooking while others such as whole fryers do not...other than when smoking, low & slow allows for more time in the smoke, so more smoke flavor.
So, I'm assuming this is going to be a turkey-day bird? It would make for an impressive feast if you can pull it off...be sure to invite plenty of hungry mouths, as it will feed 30+ with quite a bit of leftovers for sandwiches, casseroles, soups, etc.
OK, since you say you don't know turkey, let me break-down some things you'll need to consider to prepare the bird (and yourself) for this adventure...
-Thawing the bird: needs to be fridge-thawed...be sure you'll have the space.
-Thaw time: for 40lb bird - minimum of 5 days, possibly 7 or more, depending on freezer temp and fridge temp...colder freezer (assuming a fully frozen and stable temperature of the bird) translates to more thermal energy to absorb for thawing (more time), cooler fridge translates to slower thermal energy transfer (more time). Keep fridge at normal temp so your other foods don't freeze, or, from adjusting temp too high, reach unsafe, warm temps. Larger thawing meats should be kept down lower in the fridge, if possible...this larger bird may not offer many options with most fridges, so do the best you can with what you have.
-Thaw drip-pan: you'll need a large baking pan or similar (I doubt that you'll find roasting pans that big) to rest the bird in until ready for final prep. If you have a bus-bin or similar plastic bin/shallow tote it should fit that bird nicely.
-Trim excess fat: the cavity may have some fat flaps, depending on how it was cut...from the sounds of this, you won't need the additional fat during cooking to keep the meat moist.
-Giblets: assuming they are packed in the cavity, remove, and smoke on open grates for several hours, then move to a covered pan to steam...chop or process for gravy...be careful with the amount of liver used in the gravy as it can get a strong and somewhat off-putting flavor to gravies, but the heart, gizzard and neck-meat are great for adding to gravy, especially when when smoked.
-Prep for the smoke: do you have a dry rub in mind? Have you considered whether you want a crisper skin, or does that matter to you? There are some tricks to crisp the skin, but the best overall method includes finishing the bird at high chamber temp (over 400*), which most electric smokers cannot achieve (that's one reason why I prefer propane smokers, btw). You can rub the bird with butter or olive or canola oil, etc, before applying your dry rub...the fats from butter/oil will aid in crisping the skin, but works best at higher cook chamber temps.
-Brine: I didn't mention this at the beginning, as you would need a rather large container to hold the bird and solution, plus, commit the fridge space for a bird of this magnitude, so I think it would be best to focus on other methods for flavor enhancement...again, not involving an injected marinade. Yes, you can get a lot of bang from injected marinades, and somewhat less from injected brine, but then you have the danger-zone to deal with, which means higher-heat cooking to avoid possible food safety issues...so, I'd steer away from that. As for juiciness of the finished bird, I have found that monitoring for the finished internal temp is your best assurance in keeping the meat moist.
Finished internal temperature: 165*F is the minimum recommended internal finished temp...I shoot for around 170* myself, otherwise you can see quite a bit more pink/red around the bones in the dark meat. Look at the bright side of this: you won't have one of those annoying pop-up thermometers stuck into the bird, that only tells you when it's 15-20* overcooked...LOL!!!
Let us know if there's something you don't understand...there's more to add to this if, especially you want to delve into methods, such as for the gravy, which I found out last turkey-day, can really put the best finishing touches to a bird meal...I was elected by all our friends and family who attended to do the turkeys and gravy, from now on, so that's what I'll be doing again this year.
Damn...wish I could be there to see this come together!!! That's one massive bird, however, you have an advantage in that it hasn't been punched full of holes for injected broth. It can be done, successfully.