Hi baddad457,
I just checked my single rail overdrive SROD tranny (model RUD) which I took out of a completely stock '83 or '84 F150 with a 300 cu. in. six. That aluminum transmission has a 6-1/2" long input shaft (when measured from the front mounting face of the transmission), completely enclosed shift linkage which places OD in the normal 4th gear shift position, and the common 1-1/16" diameter, 10 spline input. Also, my SBF aluminum bellhousings (one from the aforementioned F150, and the other from an '83 Econoline E150 with the hydraulic slave cylinder) are identical and have a 6-1/2" depth dimension. My other transmission, the T-89 3 speed OD, has the same 6-1/2" long input and came from a '67 F100 with a six. Maybe the SROD transmission with the longer input you're referring to, is from a car?

 

Yea, the SROD bell from the Mustangs is deeper. It's completely interchangeble with the later "Cobra" bellhousing used with the Tremec 5 speeds. I wasn't aware that the truck bell was shallower, but a nice thing to know now. Among the earlier bells that fit the FE's, there was also a difference in depth between the pickup truck bell and the car application bells. The pickup bells there were deeper by about 5/8". I learned this the hard way with a big block Stang after the stock bell cracked and I tried repalcing it with a pickup bell.