Getting power out of a 63 /6?
Welcome to FABO! More power=less mileage for the most part. It's hard to get both but not impossible. Gearing is about the only way to dramatically do it. An overdrive trans with a low ratio rear is the best way. That way you have power on take off but reasonable RPM on the freeway. Cam and carb improvements can help also but are limited in what they can do for both desires. A low restriction exhaust can help also if it's done right.
this can go either way depending on how you do it... if you throw parts together and hope for the best then yes, hp+ mph down...
MPG is directly connected to VE, volumetric efficiency...
slants with there poor cyl head and compression is a great place to start...
a slight port job to increase the amount of A/F charge will help increase power. now becasue of your new increase of power you can get better MPG becasue it takes less fuel for the same volume becasue it is more effecient...
my slant with a 600cfm Mechincal secondary carb, 246 degree duration @ .050 with 10:1 comp. and a 3500 stall converter/3.91 gears get 20mpg. did it to vegas and back...
you have to build a combo for the best VE and make sure when it is operating it is in that area of highest VE.
as far as carbs a mech secondary carb can get better MPG if you can keep your foot out of it. you dont have to change a spring so it will open later for MPG, or sooner for power... with a mech secondary carb you dont have to worry about the secondaries opening unless you tell it to...
so use the 390, probably skip on the headers, you will lose heat thru the headers unlike a stock manifold... get it to 9.1:1 -9.5:1 compression and give the head a nice work over...