what combo = decent mileage+ power/torque

Gas mileage is all relative to hp. If you want more hp then your mpg goes down. A well built 360 can get good gas mileage. If it's a daily driver then reuse the crank and rods, and try to get a piston that can get you 9.7 to 10 compression if you want to run 92 but since you said you want mpg I take it that you want to run 87. If so then aim for 9 even. If it were mine I'd reuse the heads even if they have 1.88 valves and if you can port I would open up the bowls and pushrod pinch, profile the valve guide boss and smooth the short turn abit...nothing fancy just help it breath. Some people think that porting will lower gas mileage but I haven't seen, heard or showed any difference on my motors. Porting is free and you can grab some real hp with the exact same combo if you know how to do it.

Sorry this is long but the is alot to it. As for manifold a good dual plane is your best choice and bottom end power is were your gas mileage is at. Cam.. I would use a set of 273 rockers and put a solid cam in it. Something small say in the 230-233 range at .050. Again most will say you will get bad mpg but look at the slant 6. Try to get the most advance you can without pinging. Probably one of the most over looked is the carb. I like the holley or demon because they are easier to tune in my books. Try to get the air fuel ratio in low 15 at cruise and mid 12's for WOT. Hope this helps. Oh before I forget go with 2.76 gears, they are a blast on the interstate.