I would try for 9.0 to 1. Thats a good number that will run on 87 octane with killing the pistons or bearings. You can get away with more with a good timing curve, and carb calibration, but you will give up some performance for fuel mileage. At 9.0 to 1 you can achieve mileage with out giving up much.
Some of the new cars are in the 11 to 12 to one range, but the computers are able to control all the inputs and compensate as needed, your carb won't!
For the 250, a simple piston swap will get you there, for the 283, I have no idea.
Joe
