I decided to order a few motor mount plates to compare the quality and spacing. First off I want to keep my trans in the same spot so the 1 inch setback is what I need. I have a 1st gen 350 sitting at work so I measured from the bell housing to the single motor mount boss on the side of the block. That length was 16 inches.
I ordered hookers #12611 1 1/4 setback, when bolted up the measurement was 16 inches. The part was 3/8 thick and the backside was cut to fit the contours of the block. It also sets the engine lower by 3/8 of an inch.
The next set was transdapt #4595 which is a 1 inch set back. When bolting it to the block it measured 15 3/4 inches. The thickness of the plate is 5/16 and I noticed that I would have to grind on the backside of the plate to fit the block.
I have on order a set of edelbrock mounts to measure also and will be doing some test fitting soon. Hope this info is helpful.