I've never built one. But I have flown a few big Stingers.

Here's what I'd do:

Build your wing 80" and cut out the foam as you plan. Use the carbon fiber as you had planned.

Try to build the plane with balance in mind. I built my Midwest 80" Extra 300S with a huge engine. I built the entire plane with the engine- radio- servos-linkages and landing gear. But I left it uncovered. That way I could check the balance before covering it. Build it and make all your linkages. Install all your radio gear and drill out for your hinges. Tape the control surfaces on with masking tape. Cut and tape your plastic parts on. Assemble the entire plane with no covering.

Then check your balance.

You can then decide how much to cut the firewall. You may not need to cut it back if your servos are in the tail.

As for your dowel rods: Just tape the wing on and drill out the wing with a drill bit. Epoxy your dowels in and then realign everything. Get it all straight and then tape it in again. Drill out for your rear bolts and your done. If you screw up with the rear bolts-- just fill with microballoons and 15min. epoxy and try again. No big deal.

I don't know about your engine mount.

I'd use about a 20oz tank for the gas engine. It might be a bit much- but it beats deadsticking on an empty tank.

As for the plastic parts. I don't think I'd want to add the weight of the glass. You have a front and rear former for the turtledeck and you also have a front and rear former for the hatch cover. Glue the formers in and sheet with 1/32" plywood or 1/16" balsa. Throw the plastic turtledeck and hatch cover in the trashcan. Your cockpit will still be plastic- but that area seems to hold up well on most Stingers I've seen. The hatch cover and turtledeck start getting a bit ragged and ugly rather quickly on the Stinger. If you decide to use the plastic-- avoid the temptation to attatch the turtledeck with screws. Just glue the sucker on with CA and forget about it. One of my friends used 4-40 screws with small blindnuts to attatch his turtledeck. The screws vibrated out and made a mess of the turtledeck. The vibrating screws just eat the plastic up like a grinding wheel.

With that big motor up front- there probably won't be a lot left of your cowl by the time you get it hacked up to fit around the engine. Leave it off and tell everyone your flying a Stinger/Stick. The cowl will be so flimsy and weak when your done that it will just come apart and serve to frustrate you.

Just my $.02