I am in the process of building a 60 F100 and stumbled across a mysterious FE short block when I was looking at a set of heads.

9H2 for a date stamp
There is the standard 352 on the front with a 63 above it.
25-dif on the side (dearborn)
Main rod cap- C2AE
Connecting rod - C7AE-B
Crank- C4AE-B
approximate bore is 4.077
I am unsure if its any kind of truth to it but I cannot fit a 15/64 drill bit between the cylinders. one spot near the top I can force it between the cylinders in very small area.
No other spots can I even close to fit it between them.

Dont know if it matters but there is a 39 on the cam.

No other numbers at all on the block

My question - is this a standard mix master 390? heavy duty? I understand that its all factiry but interssted if any of the block could be used as factory "Higher" performance.

Also, My 1960 F100 has a very strong 352/c6 combo installed at the moment. I plan on finishing the truck before I replace the motor. Anyone remember any old school tricks to squeeze extra horsepower out of the old guy intil I finish my engine build? I would love to be able to drive it while Im worrking on the motor final motor.

Its a standard 1961 2 barrel 352

Please let me know if anyone has any questions.
Allot of information in a little space.

Thank You very much for any help