Now that everything is perfect, I want to screw it up.

I'd say, if your brother want to build an engine, let him build one for himself. If you want better MPG, change the carb to a spread bore - the primary side is alot smaller giving you air speed and the secondaries are larger to give you the CFM. I remember when I bought my old Valiant in the early 90's & it had a 350 Holley on the 318, it was slow and drank, change the maniflold to a Holley Street Dominator & bolted on a 650 Holley Spread Bore. Milage went to 27mpg & it ran 15.0 on the qtr - stock exhaust manifolds, stock cam, etc.

Trouble with going back to a 273 would be the lack of torque in comparison & you may be left very disappointed. The car may be worth more with matching numbers but if you go to sell the car, you can put the 273 with the sale package - bringing the price back to where it could be.