I'm not 100% sure, others will chime in, but here is my understandings. It sounds as though you've found motor mounts, probably side? mounts. The bellhousing flange on a 292 is the same as a smallblock. Of this I can say for sure. I assume your trans mounts are on the bellhousing, as the later trucks are. I believe the SM420 4 speed uses the same mounting pattern in the front whether it is open or closed driveline. That would make me believe that a bell housing from a 55.2 to 66 V8 truck or 63-66 straight 6 will work on both ends.
Devin