If you have the right shroud and the correct diameter fan, I'll bet your problem is motor or trans mounts. Fan should be 1/3 to 1/2 in the shroud, no matter how far that leaves it from the radiator. Properly positioned, the fan sucks air thru the radiator. To far in or out of the opening affects it ability to do that.
http://kurtspontiac.homestead.com/DavidJones69GTO.html
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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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