Frank, it does not look like you got any responses to this question. I am in the same position now and am looking at options to re-tool my clamps from 5/8" to 3/4". Some of my clamps are made by strong hand and I hoped that they made 3/4" bushings/spacers that I could use to make the sizes work. They do not make 3/4", BUT I found 5/16" ID, 3/4" OD by 11/16" long mild steel spacers at metal spacers online Mild Steel Spacer 3/4" OD X 5/16" ID3/4" / 5/16" / 11/16"
For the other clamps I am thinking about ordering an 8" long piece of the same 5/16"ID x 3/4" OD spacer material and cutting it to length. I will cut off the 5/8" nubs and then drill and tap for a 5/16"-18 tempered cap screw to attach the spacer. If that fails then I’ll weld on 3/4" solid round. This is one of those instances I wish I had easy access to a lathe/machine shop. Any other ideas out there?