Why don't you build an extended plate on the sides of the upper and sand bag them,or counter-weigh them ? Those MASSIVE springs may be effecting accuracy.
The Barrel blocks on our rails are about 6" long and most,,not all,, are torqued to specific settings. That short block may be Choking the barrel.
Are you taking the lash / harmonics into consideration ? a 1.500 will shoot differently than a .750
I M O which is reinforced by better rail shooters than I am,, 3 point is the way to go on Powell based design ( Aluminum to steel), however a mylar sleeve works well on a Young design ( Round) . NO BEDDING Compounds,,,, You're in UNCHARTED TERRITORY
Pretty rare that my rail will like the same load that my bag guns like. Just saying.