The calculation for HP to pounds of fuel is universal not a short cut it's just the physics.
1 HP = 2545 BTU’s per hour

Gasoline is 5.92 lbs per gallon
Gasohol is 6.1 Lbs per gallon

Flat head...

Ok so with the timing bypass connector connected it is reading 0 degrees
And with the connector disconnected it is reading 20 degrees
If so than you have the timing set 20 Degrees AFTER TDC...

450 HP Hour requires 9.16 Gallons per hour of Gasohol.
Mechanical efficiency of stock motor is 49% with new pistons rings increased compression and roller cam 76-80%

At 6lbs per gallon thats...

First the base timing is way to low that will make exhaust manifolds glow.
The manual says 0 and it will run like crap like I said set your timing at 3000 to 38 degrees the base timing will fall in...

C2500
The computer tuner guys I think will have you covered.
However lets look at the basics first.

You are not getting what you expected to.
First lets check the timing
Are you running pump...