I assume this car was a stock 260 car factory? I would bet something like a 2.41 open diff. The first thing I blew up with a 403 was the soft open spider gears. The Yukon replacements are cheap and much harder, slicks didn't hurt them. See what trans is under it, hopefully not the dreaded TH200. It could also be a TH250, look for a stud by the cooler lines. I toasted a high mile TH250C in one day behind my mild 403. Also look for an electrical connector on the front of the pan on the driver's side, that indicates a lock up torque converter trans. About the same strength except about twice the price for a high stall converter. Those 4A heads stock suck for flow, about 195/120, hopefully some port work was done, the exhaust side especially with a bigger exhaust valve, 1.5" is stock. If it does have the KB 6cc dish or the Arias 3cc forged, the Edelbrock BBO heads, 77cc vs you 84cc chambers, will bolt right on once the head bolts holes are enlarged. If he lied and there are 21cc dish pistons, Edelbrock is coming out with a small block head with about a 10cc smaller chamber. Both will work with your RPM intake and flow 265/190. Unfortunately Edelbrock is behind and they are months away with the new heads and out of stock of the old version. Cutlassefi, Mark Remmel has worked with them for the redesign which includes an ultra modern Combustion Chamber upgrade. Procomp also has a 85cc chamber head, not 77cc as advertised that flows around 225/190, 240 on the intake with minor port work on the intake opening. Bare is best to buy, the valves and springs are of questionable quality. Good luck, a 403 should annihilate the rear tire of the line, if doesn't, something is off in the combo. Again, probably the nicest combo I have seen in the late 70's Cutlass, I love it blacked out.
