What should i buy next for my1989 Firebird formula 305 TBI car this is what i have bought already.........

Lakewood trailing arms
MSD Blaster coil
Edelbrock 14x3 open element filter
Removed cat
Flowmaster 2 1/2 muffler

i need to also know about a good set of heads that would give me good horsepower increase and bolt right on.And good headers since i have a TBI 305 with the open element filter i dont need the cold start heat stove.

Get some headers or take on the 'ultimate tbi' project to unchoke that junk. If you do the 'ultimate tbi' stuff make sure to throw in a tbi spacer while you're at it.

Im pretty sure the hood wont close if i get a 1inch spacer.Do the ultimate tbi mods really add horse power?

The ultimate tbi mods will be good after you do more mods to the car like the headers and exhaust and intake because then you'll get the best flow through your throttle. I'd say do those mods, don't bother with the spacer, I don't think it does too much, and get some headers for a TPI car with 1 5/8" primaries and 3" collectors and 3" y pipe. You're going to want to go 3" exhaust pipes all the way back. The edelbrock TES headers are supposed to be pretty good as well as the Hooker 2055's, either way you'll be good with the headers. After you have the full 3" exhaust, then you mentioned getting heads, while you have the heads off and are putting performance ones on you should go with a good intake like a performer TBI and a better cam, a lot of people use the stock LT1 cam. Good heads for our cars are the World Products S/R torquer 305's, or if you can do some home port work then you could pick up some TPI heads for pretty cheap and port them to flow really well, like 416's for example are supposed to be really good. well I kinda rambled a little but hey it's late but hope I helped ya out a little.

ive got a 1inch spacer with my 91 firebird and hood closes just fine.