The simplest choice would be a TH 350 with a Chevy bolt pattern. It's a direct bolt-up to the 250, and since the 350 was a factory option for untold thousands of 194/230/250 engines, there are plenty of OEM parts available. It's a 3 speed with direct drive in 3rd., the same final drive ratio as the PG, but with 3 speeds instead of two. If you want an overdrive, the 700R4/4L60 (same transmission, different name) and they're also pretty plentiful, as a lot of the S-10 pickups with 4.3L V6's used those transmission.
Other options would be a TH400 with the Chevy bellhousing, and a TH 250 (not really a good choice!).
Jerry
