2.78 would be OK with 245/50/14 but I agree that 3.08 would be better. 2500 at 100kph is pretty good so I'd stick with the diff you have and spend the money on a Quadrajet and Edelbrock Performer. You may find that it uses more fuel at 2000rpm with that manifold and if the cam is too big. A HEI dizzy with vac advance will help too.
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