Plan on spending no less than $1200ish on a good converter.
Bolt togethers are nice, but not required.
Neil Chance, ATI, Transmission Specialies, Coan... I'm sure there's more quality venders out there.
Before you call them though, it would be best that the vehicle is done, and an exact weight is known.
If you give them garbage information, they can't build you a good converter.
Personally, I run a Transmission Specialties 10.5" Northstar Spragless. But, converters, like brands of underwear, are a personal choice.