Was not planning on doing any engine work on the car until next summer or beyond but my neighbor just pulled a 350 rocket out of his 68 Cutlass and said he would sell it to me for $700. It comes with everything and had 123k orginal miles on it. Sounded great in his car and was pretty fast. If I can be sure it will fit in my 90 Firebird I would love to pick it up and get rid of the 200k 305 that is in there now. Will this fit and is it worth it?

It's not even worth $200, and it's sure not worth swapping. If you pursue this, you'll need to pull the heads, and have hardened exhaust valve seats installed to survive unleaded gas, and you'll need lower-compression pistons, so you'll need the crank rebalanced, you're looking at a $1000 rebuild. Even if you decide to try to get by with lead additive and octane booster in the gas, you're still no better off than any 350 chevy, and you're blatantly illegal. And if you are going to rebuild it anyway, you may as well sell the short block and get a 403 to put those heads on. Then you're getting closer to cheap speed. But after you get the engine squared away, then you have to connect it to a transmission, preferably a 200-4R. Also, you will need to either adapt Holley's Pro-Jection 2D to the engine, or modify the car's fuel system to work with an also-illegal carburetor. There are cheaper and easier ways of doing this than the common method of installing an aftermarket bypass-style fuel pressure regulator. Then you have to find exhaust manifolds that will work, and have a new custom Y-pipe made by a local muffler shop, so you won't get high-flow mandrel bends, you'll get restrictive crush-bends. All in all, lots of hassle and expense for no good reason. If you want a 350, do a chevy. It's easy, it's cheap, it's legal, and it's at least as powerful.