I've used a trans crossmember from a first series Camaro before, with a small amount of adaptation it's nearly a bolt in. Unless you can find one cheap, which is harder now because of the popularity of these cars, here's a store bought option http://www5.mailordercentral.com/rbs...item=1&mitem=1
Year One, and other Tri 5 chev parts vendors offer them too.

Now, something else to think about. I'm going to assume you have the stock 283 block, and stock side mounts on the trans bell housing since you didn't say. If you put a TH350 on, and go to the rear (relative to the trans) crossmember, you won't have those bellhousing mounts. Since your stock engine is mounted on the very front you've got quite a span between the front and rear supports. It puts a lot of strain on the bellhousing/engine block mounting flange/bolts. I'm not going to tell you it hasn't been done, and some have gotten away with it, but many have cracked their 350 trans case too. The fix would be to mount the engine with the side engine mounts of the later engines, but as I said, I'm assuming you have the '57 block which doesn't have accomodation for those. Now, if you've got a later block that has the bosses cast on the side for the mounts, that would be the ticket.