Quick update on my '98 K3500. After talking with usanumber1 a couple weeks ago, I ended up buying some 1" plate steel and making a spacer to go between the block and the driver's side mount. This got the SSOR mounts really close, but they were still a bit too far off. I would have had to elongate the holes in the frame/mount pad to get anything to line up properly. I also was not happy with the placement of the engine in the bay with any spacer between the block and the mount.
Paul (SSOR) did reach out to me and offer to send me some spacers, which I accepted. They ended up being 1/2" spacers intended to use between the frame pad and the mount (I, of course, removed the 1" spacer I was test fitting before trying these).
A couple friends came over and we spend the next 2-3 hours trying to fit things together and get everything sitting where it needed to. The bolt holes were still a nightmare to line up (as others have reported). We eventually got all 4 bolts in each mount into the frame and thought things were looking good. It wasn't until we set the weight of the engine down on the mounts that we realized we still had a problem. The engine was sitting too high and (a) the oil pan lip was now sitting on the differential slightly and the bellhousing was contacting the firewall/tunnel.
At this point, I was fed up. I've messed with these $400 "bolt-in" mounts for WAY too long. After spending hours and hours of my time trying to fit them, reading about the experiences here, and discovering more of these same exact issues from folks over on the GM Full-Size Diesel Swap page on Facebook, I called it quits. I'm done with these mounts. It does not matter how awesome these look and how strong they might be if they don't fit. They are NOT worth the hassle, headache and heartburn. SSOR's product needs a bit more R&D before they are ready for mass production, especially for the price. It's a shame; it really is, because they look freaking amazing, but it's the reality of the situation.
Now, for a path forward so I can FINALLY get this damn engine mounted in my truck. I called Joel @ Jonesy's. He answered all of my emails QUICKLY, addressed all of my concerns and gave me options to help me with my project timeline. I ordered a set of his conversion mounts, which arrived this evening. If the 1st gen style mounts move around a little bit, I'll cross that bridge when I come to it, but for now, these will get me moving towards getting my truck back on the road.
I'll update as I [hopefully] make some progress this weekend.