Most of the new "aftermarket" motor mounts (not the poly performance type) are made in China. The rubber is softer, not as strong and not up to snuff compared to the mounts of yester-year. You add some weight, a good bit of power and a bunch of header heat and you have a recipe for a sagging or torn motor mount. The thru-bolt trick will work until the mount starts sagging and deforming. After that the thru-bolts are pretty much useless. My solution was to fabricate some solid mounts from a set of Chinese specials. Hell, this car is all about NHV anyway... An added bonus of solid mounts is you can rigid mount your sidepipes too.
Here's some pics I posted over on Lone Star's foum;
Here's a new mount torn in half with about 500 miles on it.
Here's where I trimmed the rubber around the perimeter to enable welding the 1/8" x 1 1/4" strap steel around it.
Here's the finished mount. All totalled, I've got about 2 hours and 50 bucks in the new solid mounts.