How much power do engine mods yield?

Back to the OP's question....and the rear gears. I have a '62 Dart with the stock 3.31 rear gears and it is turning over 300 rpm at 70 MPH, and is probably edging into the power valve some. It is not all that great for a cruising combo. I have a 2.94 rear gear set that I'll eventually put in and live with the lower wheel torque. But this car is a grocery-getter, not a hot rod, so what the OP wants is the most important.

So if the OP is going be doing a lot of interstate driving, and has an auto trans car it probably has 2.94 or 2.76 rear gear and it would be good to stick with that for interstate use. If his driving is all in town, then a steeper gear would be nice. For mixed town and highway, then I would go with a steeper rear (like 3.23, and maaaaybe 3.55) and convert to the 4 speed OD trans to help keep the interstate RPM's down.

BTW the original list of mods looks good per wide experience, with the dual exhaust being for the header as said.