Rebuilt engine lacking power

[MJN]

I rebuilt the original 350 in my '74 K20 and got it running this spring and it runs great but going down the highway at 65/70 mph it seems to need a lot of throttle (at least 1/2 throttle) to stay up to speed and usually need to floor it to get up hills. Even when I floor it to accelerate up a hill there doesn't seem to be any difference, I will lose speed or stay constant speed up hills. So far I've got around 750 miles on it. No mis-fires, exhaust doesn't smell rich, no soot. I haven't pulled plugs or changed fuel filter yet, doesn't seem to be starving for fuel.
It got bored .030" over and I put flat top pistons in it with a Melling MTC-1 cam (Intake Duration at 050 inch Lift: 204
Exhaust Duration at 050 inch Lift: 214
Duration at 050 inch Lift: 204 int./214 exh.
Advertised Intake Duration: 278
Advertised Exhaust Duration: 288
Advertised Duration: 278 int./288 exh.) Otherwise its a stock rebuild, factory intake and exhaust manifolds, 3998993 casting 1.94 heads. I had the quadrajet professionally rebuilt. I have a Mallory HEI distributor and set the timing at 10-degrees BTDC (yes I had the vacuum advance unhooked and all vacuum ports plugged). I've been running 93 octane, no ethanol gas in it. Not sure where to start since it runs so well otherwise. I can light up the tires and it has great pickup and acceleration but seems to just fall on its face going down the road.

The truck itself is a '74 K-20, 4.10 gears, 4-speed manual (SM465) and I have 37" tires (actually measure 35") on it.