I concur with Lynn - 2400 stall and the 1103 kit would be a good fit with the TH350 with the 3.08. Won't see God on launch, but will definitely notice performance improvement over stock!

For springs, I'd go with "Z28" type springs - they're readily available for well under $100 - key is to stay at or below 325# open to preserve the integrity of the cam and lifters.

Check this link

Z28 Valve Springs, Retainers & Locks sb Chevy 265 283 305 327 350 400 sbc | eBay

Regards,
Glenn