I use an X block on my bloomer 4.250 w/ Indy 360-1. I shift at 6500. It’s been 5.66
My R block molnar 4.125 W-8 had been 5.53 at 3000’ DA shifting at 7200. I might be a little afraid of a stock block, if I used on I’d sure use a girdle or something to beef the bottom end