The 4bbl sprint engine had more compression, Valve sizes, I believe were the same.
I am pretty sure the cam was a bit more agressive than the 1bbl version?
The 4bbl had split 3& 3 exhaust manifolds.
Camshaft failures,wearing too soon, oiling problems possibly?

MBHD
Ahh, here we go.
http://www.overheadcammerschapter.150m.com/history.html
The Sprint option included a high lift-longer duration cam, dual valve springs to control valve float up to the 6500 redline, 10.5 to 1 high compression head, special harmonic balancer, dual exhaust manifolds (3 cyl. each to control exhaust pulses), high flow intake manifold, Rochester Quadrajet 4 barrel carb and a V8 clutch disc and pressure plate

12 port SDS EFI