I have a leak down Gage that I use to check the condition of airplane cylinders. I plan to use this Gage on my 7.4MPI and I am wondering if their is any special instruction for top dead center on each cylinder or a certain reference pressure, (80#s perhaps).

I usually use the rotor in firing order.

I use the distributor as an indicator. It will get you close and you ca use a breaker bar to spin the crank a little if you're off and getting air by a valve.
I've seen guages that range from 30 - 80 psi testing pressure. I prefer the lower since it wont push the piston down if you're a little off TDC.

I pull the valve covers off and pull one plug at a time and tap the starter with a remote starter. When the compression tester tube is attached to the spark plug hole it will suddenly press the air out and give enough resistance for you to get to TDC. Now if you have a degreed balancer even better.

Every 90 degrees is top TDC starting at any cylnder you start with.

I prefer to start at 1