The "kit" would include a complete 55-57 Chevy rear end and driveshaft and standard transmission. Now hook up your 216 or later 235 and you're ready to go. Alternately and easier to find, a complete 250 six cylinder drivetrain from a 67-74 Nova would be a good swap, either standard or automatic. I helped a friend put that combo (with automatic) in a stock 37 Chevy in one weekend back in the early 80s. The exhaust etc was a combo of stock pieces and the car was driving the next weekend.
Another thought; I believe it's either Patrick's or Langdon's that sells a kit to install a Saginaw 4-speed and KEEP the enclosed driveline. But, they need your mainshaft from the Saginaw to machine/modify it first. You'd also need a different bellhousing to bolt the Saginaw to the engine. I am Butch/56sedandelivery.
Actually the 330 HP 350 would be a better swap out of the same Nova. LOL Those old 10 bolts are cheap and plentiful and the Nova should be about the right width.