360 driver build

so i read on a different forum, cant remember where, and they were talking about how to build a 360ci for high performance an get in the ballpark of 20mpgs. someone there said to build it to stock specs of a 340 for 70 or 71. they also mentioned getting J heads with a 1.88 intake instead of a 2.02, saying you dont want 2.02 for a street car.

so i was wondering why that is? y would one choose a smaller valve diameter over a larger one for mileage/performance. i want to do the same thing, build my 76 cordoba 360 to get about 20mpgs an 300+hp once installed into my 70 dart.

if anyone knows or has done this build with positive results, please post any numbers or brands i should look into. i know pretty much everyone is, but im on a budget of...well i dont even know yet! so the cheaper the better as long as its still reliable

thanks!