Realistically, if you don't plan to remove the roof for display I've found it is best to glue it on to get it straight. Also worth noting is that the hinges for the rear hatch aren't the greatest. I find them oversized and fragile.
I've done the same firewall modifications and completely swapped out the engines. The kits block is just too funny to make look right and the SM465 looks better with a 1:24 engine, IMO.
https://public.fotki.com/fabrux/blazerjimmy/