School me on torque/stall converters

so im buildin my 70 dart to be a good performer that wont absolutly suck down the gas. A long haul driver that i can eventually take on the Hot Rod Power Tour. since gas is rediculous, i need to save where i can. i know the tranny can make or break mpgs and fun.

So lets say i have a mild 360ci w/ a 4 barrel and TTI headers and true duals. in my 904, what stall would i want? how do stall converters work?

basically im askin an age old question the long way: How do i get a happy medium of quick and badass muscle car with decent MPGs so i can actually enjoy the drive instead of dreading the pump