The early closed chamber heads
should be about 10 or more cc's smaller than the low-compression smoggers on your 74 engine. This will bump your compression up from a true 8/1 to a true 8.8. and your cylinder pressure up to 158psi@sea-level.
Even if your cam is stock, The Dcr on this new combination is just 7/1, so it should run on 87E10 at full timing.
If your cam is not stock, the later ICA will only reduce the pressure, reduce the Dcr, and that means for sure it will run on 87E10
If you are at higher than sealevel, the pressure will again be reduced.
All-in-all, I would rip the bad head off and have a look-see.
If your pistons are where they're supposed to be, down in the holes around .157 inch, you should be good to go, AND
if you find 020 steel headgaskets, that would be a good indicator that the engine is still original. If it was mine, I would install the thinnest headgaskets that I could find.
However, in the event that your 71 heads have 60cc chambers; this will increase your Dcr to 9.14, the Dcr to 8.1 and with the stock 48*Ica, the Pressure jumps to 166 at sealevel. This will require Premium gas, for WOT operation; but the reward is excellent power.
I would still do this build, but with a thicker head gasket.
The bottom line is, that you won't know for sure until you get the heads off, and do some measuring.
And before any calculations are done, we need to know your local elevation, at which this vehicle will be operated at, and the Ica (Intake Closing Angle).
As for the Ica, without taking the front of the engine off, and degreeing the cam, here is a pretty good approximation;
With the engine fully assembled;
1) take all the spark plugs out, and using either #1 or #6 cylinder, whichever does NOT have a bent valve, roll that Piston up to TDC on the compression stroke, then back it up a half a turn.
2) we are gonna need to find fairly accurately, at what degree your intake closes, which is gonna be somewhere between 45 *After bottom dead center, and not more than about 70* ABDC; so to accomplish this, your balancer will need to to be calibrated. The best way to do this is to install a timing tape.
But, there is another way, that is accurate enough.
3) Install a fitting in the selected cylinder hole, 1 or 6, and inject about 20 psi into the cylinder. Then put a bar on the crank, and begin to rotate the Crank Clockwise, until you get resistance and most if not all, of the hissing stops. Find the exact spot of transition, then put a mark on the balance, with a sharpie, adjacent to the TDC Timing mark. If 20psi is too hard to work with, crank the regulator back until you can manage.
4) Rotate the crank back to TDC Compression of the cylinder you have selected, either 1 or 6, then measure the exact distance from TDC back to the mark you made with the Sharpie.
5) finally, measure the diameter of the balancer to the nearest 1/16 inch.
From this work you have done, I can easily back-calculate the Ica, within a couple of degrees.
If you cannot do this work, or areunwilling to make the effort, no worries;
just do it the old-fashioned way, namely, bolt it together and see if it detonates, on Regular gas. If it does, then try best gas. If it still detonates, take it apart and start over.
But if it does not detonate on best gas, try next lower octane.
