T&M Time and material or cost plus is about the fairest way to do things. Tell your client that you have a minimun of X amount and an hourly of Y amount I would push for $200 minimun and $35 to $55 hourly depending on what kind of car his wife drives.
Be honest and accurate about your time. Include your time to the store for supplies, set up, clean up, bookkeeping, travel to and from, everything that takes time away from your life for this project gets billed to this project.
Keep all of your receipts for this project and buy everything new for this project and prorate the leftovers for the client or surender all unused consumables to the client. Using your half can of paprika out of the pantry is thrifty, but you need to get paid to replace it, keep that in mind.
If you have 10 hours in this project and you charged $35 an hour that puts you at $350. I would do it for $350 and the fun of doing it. Keep in mind though, this is also prime time, the weekend and that should be reflected in the pay.
Ultimately you will have to do your own negotiating and evaluating the profit/loss end of it all and compare it to your standard.
As for stacking ribs? Not in contact like laying atop of each other, the smoke and heat would have a hard time working its majic. Rib racks are another Idea but I am not familar with your rig and I know that some racks seem to work better in some smokers than others just because of size and design charactoristics (SP)? I am an advocate of more cooking grates/grills/levels. Maybe double you cooking area by using bricks or the likes to support another level of grate/grill over your existing level.
At any rate, good luck and keep us posted.
Oh yeah, post pics if you can, we all like it when a plan comes together!