Wow, this guy just gained a customer here from his email response to me. This answers a lot! I asked if he had the thicker bed mounts that are about a half an inch, like what I need for my panel vs the 1/4" pads that I got apparently for a pickup application. This is what he said:
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Hi, Chuck
The 1960-72 Suburban / Panel truck body mounts were last available through GM up until about a year ago when the last available were finally sold. These are the mounts you need, and when new they are actually more like 3/4"-1" in thickness not a 1/2" and have a steel sleeve molded into the bottom cushion that is slightly taller than the cushion and the top cushion has a small depresion in it that the round steel sleeve fills when they are mounted and tightened. We have heard that there is a company having a manufacturer reproduce these and we expect them out sometime this year but it looks as though it will probably be late summer or sometime in fall at earliest before they become available.
Have a nice weekend!!!
My Very Best
Paul @ GMCPauls
Online Catalog-
http://www.gmcpauls.com
GMCPauls Truck Parts
505 Adams Drive
Rockville, IN 47872
Fax: 773-442-0103
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That answers a whole lot, as my mounts must be really smashed down and rotted! I think this amount of lift would fix my problem, or at least allow the bed to sit level. I still think that the one cross sill in question (that is currently sitting at the high point of the frame over the axle) would be touching the frame with no room for any rubber mount, however. I still think that it had to have been mounted in a different location, like one set of "skid rail" bolts back.
I did see a few sources that had mounts that looked like the ones I needed, but they did say 67-72 Suburban I believe. That was only one place out of all the truck restoration parts suppliers that I had looked into.
Due to it not being a leaf spring rear, and the possibility of towing, I still think I will try and take a piece of 3" structural steel channel and weld it to the bottoms of the rear frame extensions, and then where it comes up towards the axle, and the frame kicks up, I think I will take some 1/8" or maybe 3/16" plate steel, and add a side gusset to the frame, from the bottom of the channel and following the contour of the frame kick-up, with as much hanging down below that as I can without risking having the axle tube bottom out on it. Might do the 77-81 Z28 trick and throw an aluminum spacer block under the bottom-out bumpstop rubber snubber.
I feel pretty safe that if I had all new body mounts in place and all bolted in, that the body itself would lend a huge hand in stiffening/strengthening the tongue weight capacity of the truck, but I still may do that reinforcement. I rarely tow, but once or twice a year is enough to justify the extra insurance. My buddies ask me now and then to tow home a new lead sled project for them, and my 71 Camaro project might need a new storage place to call home soon when I move...