Here are afew things I'd check if I was you!!! ...IMHO
-Check and see what head you have, GM casting the 049 & 781 are the best (perf. combustion chamber) Then I'd add over size vlv.s, hardened seats, & stiffer vlv. springs. NOTE - 454C10 has it right! "BBC's eat cams." I use a flame hardened cam in my engine.
-850cfm carb is too large for a street driven machine, you'd probably do better with a 700 to 750 cfm carb.
-HOPE who ever built your engine used after the market rod bolts!!! Stock rod bolts are abit "stretchy" with a H.P. motor!