Bob,
About 5 years ago I bought a '52 GMC 1/2 ton with a late model 235 installed. One funny thing that I noticed was that when I drove over bumps on the road, the whole driveline would "bounce" and oftentimes the 4 speed would fall out of gear. I thought that I had a worn out tranny but it was never really that bad so I kept driving it.
A few months back I pulled out the 235 and put back in a GMC 6 cyl engine. As I pulled the 235 out I saw a possible cause of the driveline bounce. As you know the 235 is about two inches shorter than the GMC block and the PO had used a flat piece of 3/8" steel to bridge the gap. Two bolts went through the frame and the other two went through the timing cover and engine mount as usual. He had also put a piece of flat steel of the same thickness underneath each engine mount off the sides of the bellhousing. I think this was so that the engine was raised the same amount in the front and the back.
One thing I noticed was how much the steel plate in the front allowed the front of the engine to flex up and down. This could have been because of worn/loose bolts or bad rubber mount, I'm not sure.
My thought was that when I would go over bumps in the road, the front of the engine was bouncing up and down enough to put a bind on the driveline.
This theory could be bogus but I do know that with the GMC engine sitting on the crossframe like original, I don't have any more problems with the same tranny falling out of gear.
Because of all this I would recommend that whatever method you use to make the 235 fit on the front crossmember, be sure to make it solid and strong enough to support the front of the engine weight.
Hope this helps.

"The Operation of GMC trucks is normally smooth and steady, with no loud rattling, knocking or unusual noises. A good driver will quickly become accustomed to driving a GMC truck and will get the feel of his vehicle regardless of his engineering knowledge."-TM10-1563