A tiny bit of progress yesterday. After spending quite a bit of time shifting the cab around to just the right spot, I drilled out the frame for the upper mount cushions to sit into, using a 1 1/2" hole saw. I'm also planning on drilling a 2 1/2" hole on the inside only of each frame rail to access the bottom cushions of the mounts. I decided to go with the 67-68 style mounts. I'm doing it this way to help with ride quality. Mounting the top section only isn't much better than just mounting it to the cab. Need to have a bushing in the middle to keep it from flattening, etc.
For others looking to do the same, I found that the measurements for the front mount holes from the original slots in the frame are 3/8" forward from the front edge, and 1/8" inward from the hole's center. See pic where I measured it out. Blue dot is where the center of the new hole is.
I then measured up and cut a notch on the tank mount for the left rear mount. Planning to build and weld a cab mount to the top of the tank mount.
