The quotes you got are in the right ballpark. You can go way over that if you want to. I'll bet you are way closer to the 5K than the 3K when its all done. The plan I have for my 350 will probably push 7k. The #17 heads are an excellent 350 head. I would have them redone including adding screw in studs. I wouldn't worry about the valve reliefs. The question is what is the compression? If you need all new pistons etc you will be spending $500 for forged pistons. There are some other options and you can find plenty of chatter about 350 pistons. I wouldn't get into reusing the 74 pistons. For all the money you are putting in I would overbore and work with your engine builder to pick a solid piston. Lot's of 350 performance street builds use cast pistons. Others hate them. I have no idea. Others are using mopar pistons. Or get custom. We can really get into the weeds here.

You can certainly reuse that 4bbl iron intake. It will work great. But don't over carb it. Pick a good cam and add duel exhaust, if you don't have it already, and you will be well on your way to a very nice set up. With the right gears and posi a built 350 will move a little Firebird with plenty of getty up.

Now you are bound to hear, if you haven't already, that for the same money you can build a 400 or 455 and get way more power. Totally true. No doubt about it, 100% correct. But if you are looking to keep the original 350 don't be afraid. You can get 325hp easy.