If the cam has under a 215 duration @ .050" I would also agree that a stock convertor should work. Putting the convertor question aside for a moment something else came to mind and is similar to one I had with my 250 only I'm running a 4 speed. Trying to get a steady and controllable idle was a pain until I noticed a little sentence in an Edelbrock carb tuning book that said some engine / cam/ carb / ignition combinations do not like to run with ported vacuum advance and the solution was to run the vac advance directly, meaning from the nipple at the base of the carb.
Deciding that anything was worth a shot I moved the vac line to fully ported and everything fell into place. It sounds contrary to what we know but it works fine. Idle is great and the carb sets up nicely. I haven't had to so much as adjust the idle speed in almost 2 years. Part of what may make this work is my HEI I built using a 20 inch vacuum diaphragm that also has the lightest spring in it, so it responds instantly to the slightest drop in manifold pressure as the throttle plates begin to open.
Back to the convertor - a 12 inch convertor is pretty tight and just might be too tight for what you have. Stall is related to engine torque and vehicle weight as well as rpm. What worked on the stock 230 now will work differently on the
hi-po 250 you have. If you can check out either a listing for those PG convertors or can see one from something like a 300hp 327 you might find out there was more than one that went with the PG depending on which engine it was behind. Whatever you do about changing it don't get a replacement convertor with some silly hi rpm stall speed. You'll hate it for everyday driving.
I personally would swap to a nice street built TH350 if you get to the point you want to yank the 'glide. You'll be amazed what that one extra gear will do to wake up the 250. Summit, Jegs, and all sorts of other speed suppliers have them for around $600 to $700, complete with the right convertor.
