Rick,
There's not enough clearance on the back of the mount (engine side) for the bolt head and washer. I was going to do this. I might have enough clearance if I grind the "retainer" bolt head a little thinner. But ... I will lose bolt structural integrity doing so. It was a consideration about "wallowing out" the bolt hole in the frame. I honestly don't think it will happen though. I lot of engine bolts use a similar "through bolt" through their motor mount bracketry and they suvive for 100K miles and beyond with the same forces working on them. The elasticity of the motor mount will serve to minimize torque on the bolt/frame too. The wall tickness of the frame is substancial. I will monitor the though bolt and frame for movement/wear though.
The tranny mount's fine.. looked at that (along with putting a wrench on every known nut/bolt on the car) while I was doing my "winter overhaul".
Rick, the eternal pessimist..
Keep me honest buddy. You're talking to a guy that welded a Datsun rear differential mounting strap spring to the back of a cast iron Corvette diff housing to mount it in his Datsun 240Z (behind a 406 Bowtie small block motor)... and .... it held thru many 1/4 mile runs and as a daily driver.
Dave