318 rpm drops to 400 in gear WHY???

Posting for friend who's 72 318 dart just rebuilt motor and it idles perfect @ 1000 rpms until you drop it in gear and falls on its face to 400 rmps . Things we have checked ,vacuum leaks, cant find any . Vacuum check say 8 pounds Timing at 32 brand new Holley 650 the second one we tried still does the same thing things done in rebuild include msd ign 6al distributor, performer intake 4 barrel, hooker header ,484 lift cam , ported iron heads, 2500 stall converter . Blocked the power brake booster off and no difference ?? So what should we check next ?? New fuel pump and line also .Any ideas or help please will be appreciated. Thanks