I know you want to keep the 413 but in reality you could buy a rusted out P pumped 12 valve truck for $5k or less and swap the engine and trans and be money ahead even with a trans overhaul and good converter.
It's not that I have an affectionate attachment - it just runs great and I really don't want to remove the front end to cherry-pick it all out.
My vain hope was to jack it up and do a trans R&R, which I've done, and (besides the shaft shortening) is a few hours.
I also did got a reply from Randy (mrrandyj):
"In my opinion you should use a Gear Vendors overdrive behind your 727. You can get a towing 12" torque converter that will not slip much going down the road. If you really must have a lock-up torque converter, you can find a 1978 big block 727 case and convert it to lock-up using a small block lock-up 727 for the parts you need. That way you won't need any expensive adapters to get the transmission to bolt to the engine. With a lot of the adapters you cannot use the larger size torque converter, it will not fit. If it was mine I would use your existing transmission, buy a good non lock-up torque converter, and install a Gear Vendors.
Randy"
and my response was:
"Thank you Randy,
If I find a good used GV unit I'll consider it, otherwise $3-5k for any of these solutions will never be worth it in gas savings..."
