Saw your thread on RamchargerCentral, and answered there, but you should be able to get a wrench on those bolt heads; I don't remember a "cap" on those bolt heads. It has a small plate under the two bolts, but mine did not had a bent-up keeper on the two bolts, on my '73. As I'd said over there, I'd put new mount bushings in, while it's out.

On the transmission crossmember, mine is a bolted-on situation, also, so I'm guessing that's a factory thing. How is the tailshaft mount? Mine was bad, and the engine/tranny had torqued towards the driver side, to the point that the tailshaft mount was vibrating a notch on the crossmember. I got a new mount, loosened everything up, and straightened the whole thing out. I still have vibration that starts around 42 mph, so something is still not right on mine. Stub shaft was balanced with the other 2 driveshafts, along with new Spicer u-joints, but I've read that it does not take much to get the stub shaft to cause vibration in these trucks.