Don,
You might consider a GearVendors Overdrive unit behind that Muncie as an alternative - here's a link to the one they push for your application Gear Vendors under/overdrive auxiliary transmissions Hot Rods, GM 4, 5 and 6-Speed Manual Muscle Car/Street Rod/Sport Truck.
I was totally unfamiliar with them until Dave Severson posted his thoughts about using one in his '57 Chevy project, but after doing some reading they seem to be a very viable alternative - not cheap, but a good option. Good luck on your search.