Matt,
I did not do the 5 speed but, I did a New Process MY6 NP440 4 speed overdrive. I followed the article from
Classic Trucks which was written by
Brad54 (I think that's his Name from the forum).
It bolted on just fine. I only had to have the input bearing turned down and have a new driveshaft made. It replaced my SM420.
The overdrive is .73 and pretty well suited to the V6 I feel.
They can be tough to find and you need to get a complete unit with the shifter and rods and mounting bracket.
Another possible option on a 5 speed is the AX15
http://www.67-72chevytrucks.com/vboa...highlight=ax15