Is it rebuilt to stock spec's? If you've increased the compression you may need to drop the initial timing back a point or two. Does it have the factory exhaust manifolds? If you've changed something there the starter may be getting hot causing a slow cranking issue. And the starter may just be getting tired. Good luck.
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frittering and wasting the hours in an off hand way....
1969 GTO, 455ci, 230/236 Pontiac Dude's "Butcher Special" Comp hyd roller cam with Crower HIPPO solid roller lifters, Q-jet, Edelbrock P4B-QJ, Doug's headers, ported 6X-8 (97cc) heads, TKO600, 3.73 geared Eaton Tru-Trac 8.5", hydroboost, rear disc brakes......and my greatest mechanical feat....a new heater core. 
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