The following link should help. All chevy and gmc trucks had this bolt pattern from 1948- 1967 ish. Passenger cars also got this pattern sometime in the 1950s. Gm continued to use this pattern through the 80s and 90s on certain moldels like the camaro, firebird, and s10 pickups. Some cars and trucks had the same bolt pattern, but a larger hole in the middle starting around 1967.

http://www.allstategear.com/Breakdowns/Chevrole_%20Muncie_4_speed_Bolt_Pattern.jpg

I assume this view is looking from the bellhousing toward the transmission... You will likely have to flip the measurements from left to right to get the bellhousing measurements.