Nice find. Nothing like old iron! I love it.

Your mileage does not sound too far off. Let's allow some room for possible human error. I am not doubting your mathematical ability but just for conversation sake lets say your are off a little bit. In the case of your platform that might only mean 1-2 mpg but that puts you right around every 460 every made. You are not far off to begin with because 460 and decent mileage do not ever go in the same sentence.

Maybe it isn't you and it is the truck. Meaning the odometer is off a little bit of a percentage and you actually went further than your odom is telling you. This is very easy to happen even if your tire size increased only slightly. If it increased slightly then your odom will read slightly slower than actual.

Test it. Get on the interstate and watch it when passing mile markers. You will know real quick if you keep passing mile markers and your odom has only changed 9 tenths or a little less than 1 full mile.

If you pass and only read 9/10 of a mile then you are 10 percent off. If you pass and read 9 and 1/2 tenths then you are 5 percent off.

Look into that speedo/odom and you might be surprised how far you actually went.

Good luck

P.S.
What is your idle set at? (RPM)
Timing?
I think the tires you are running are a tad bigger than what came with the truck by about 1-1/2 inch. That would cause your error in MPG calculations.