I can tell you the poly replacements we used spaced the core support a little higher than the original 2wd version we removed (all 72 rubber stuff). This set our shroud a tade higher but still cleared our fan. Ended up shimming the front cab mounts to make everything come back in line. Worked out OK because we needed this to help clear the transfer case anyway. I'd leave it high till you can mockup everything up on your frame, cuz you can always shave um later.
http://67-72chevytrucks.com/vboard/s...&postcount=365 BTW we might have to add even more cab mount shims to allow for TC flex before it's all done?