Choosing a stall speed.

was the old converter an aftermarket one? That could cause a big change. If they went from the factory 10.75 to a factory 11.75 then that as well will knock off about 400 rpm of stall. Check you brake stall, hold your brakes hard and see where the converter revs to steady on the gas. Factory should be 2250-2450 rpm on a 70 340. If it's stalling 1800-2000 then it's probably got a 11.75 converter from an early 70's 360. you might be able to see in the inspection cover how much of the starter ring gear is exposed compared to the converter bell then compare it to pictures of factory 11.75 and 10.75 converters. It's also possible its stalling even lower with a bad converter so check where it is stalling now.
Either way you probably want an aftermarket converter which I guess is what you asked for in the original post! Sorry.