Thru '72 the trucks with manual trans had a cast iron bellhousing with bosses cast in on each side. This is where the mounts go that attach the bellhousing to the crossmember. The bellhousings thru '67, I believe, had a smaller trans pilot hole for the SM420 and the 3 speeds. IIRC it was 4-13/16", then starting in '68 when the SM465 was introduced the trans pilot hole was enlarged to 5-1/8". Also, the starter bolted to the bellhousing, instead of directly to the engine.
Here are some pics I found doing a google search......
