What about supporting the tranny? The old bell housing also bolted to the rails, but the new one doesn't have that option. Any pictures you guys have would be helpfull.
You need a different bell housing. Look for a '55 to '59 V-8 truck type for the easiest conversion, though if you avoid the hydraulic clutch models up to '67 will work. The mount pads will be wrong but can be adapated.