It's not as easy as putting "a bolt through the mount to make it 'solid.'" The problem is (if you have a 67 or 68) that the bottom of the motor mount sits flat on the frame part of the mount (point A on the figure). If you place a bolt though it, it will hit against the mount and therefore not fit. I found this out when I tried to turn my new set of motor mounts into solids (had broken 3 sets so far

). I ended up fabbing my own little design of a solid adjustable motor mount.
Here is a very rough drawing of what I ended up doing. It requires more work, but I have yet to break a mount, and had NO increase in vibration transmitted from the engine to the frame.
You get a ½ bolt or whatever size you would like to use along with a nylon nut. Drill a hole though the mount to allow this bolt just enough room to fit (the rubber between the mount will make it a tight fit as it won’t drill out all the way). Cut the head off the bolt and any extra so that the bolt sticks out about ½” past point C on the mount. Place the bolt though the mount so that it is recessed about 1/16” to 1/8” past point A. Then with a MIG or TIG welder, weld the bolt to the mount. Grind any bead that bulges out so that point A on the mount is flat. Install the nylon nuts on, but DON’T TIGHTEN THEM!!! This will come into play when you go to install your new mounts and will need the mount to stretch a little to fit. Once the new mounts are bolted to the frame and engine, tighten the nuts down until sung.
The beauty of this design is that the more you tighten the nut, the more you compress the rubber between the mount, and the less flex allowed in the mount.

The bolt and the weld are very strong, because they are placed under tensile force, which both the bolt and weld can withstand the best. The other advantage is that you still have rubber between the mount, so vibration is prevented (as I said above, I noticed NO INCREASE IN ENGINE VIBRATION TO THE FRAME!!!) If you want it tighter/looser, just tighten/loosen the bolt with a wrench (which will fit, meaning you don’t have to take the mount out).
This mount is very strong, provided that you laid down some quality welds. If you have the tools and the ability to do this, then this is a great way to make a very nice solid adjustable motor mount. Whada think???
A=Top part of mount
B=Rubber
C=Bottom part of mount
-Shaun