I've suggested this a few times; use the 200 4R, from 86-87 Monte SS if you can find one. They can be beefed up to handle a lot of power, or work fine for a stock L48 300 horse. Lock up converter to boot.
Benefits - overall length just like the TH350, same driveshaft yoke and length. Downside - dual Chevy/B-O-P bellhousing, but you can cut off the extra ears if you want to get sneaky. A good friend of mine has one behind his 400 Firechicken, and has nearly 10K miles so far with no difficulties at all, I've got one stashed for my L48 now.
Cheap cores at the yards - $25-75.00, generally.
Just my opinion -
Regards,
Steve