The only testing I've read about was reported in the piece in the Precision Shooting article on Bruce Baer, some time ago.
As I remember, he found that a block of 8 inches or less was preferable, even on a HG.
To show that short block can work, I have a point-blank BR rifle that has a block a bit more than 3 inches -- P-B rules state that a block must end no more than 4 inches from the boltface. The barrel is glued to the block, it is a 30-BR chambering, 25-inch barrel. Works just fine.
I've seen a number of 8-inch or shorter blocks used on 1-K rifles. I currently have one on my 30-caliber LG that is 6 inches long. Tempted to go to 4 or 5 inches If I redo it.
Well, not much general information, hope it helps.