If the alternator is new and system voltage is normal, that's out.
Even switching to your friend's entire distributor blew his module, so the dist internals are out.
Eliminate the external circuits/components - that's all there is left.
First disconnect the tach if there is one. Try it and see.
If it immediately blows another module, the next step would be to pull all the plug wires from the cap. Crank for a few seconds, replace one wire and test the module by checking for spark at that plug/wire.
If you get spark and it stays for a few seconds, try adding another. It will be a long trial and error process to find out what wire(s) is /are causing the issue.
But other than the tach or an over voltage situation, some problem in the secondary is all I got.
Found this while thinking about what else could you try to change:
MOST COMMON CAUSE, apprx. 99 percent
IGNITION COIL IN THE CAP. Epoxy filled coils in HEI caps are a heat stove, they do not leach heat out of themselves, as an oil filled coil does, and, this heat leads to "layer shorting", when the coil literally melts the insulation way from the individual layers of primary windings. As the windings touch, the change coil resistance on the primary side radically increases, creating massive load on the HEI, MSD, or other module, to failure. This can be gradual, exponential, or, immediate failure.
I have seen layer shorted HEI coils take a brand new, quality HEI module right out in less than 30 seconds flat.
To properly test both an HEI module, and/or HEI coil, resistance checks are no longer effective, unless both are full on DOA. Both parts will need to be run on an off vehicle run tester, most better auto part stores have them. The "tech" there will most likely want to test the parts for 5 seconds, decree they are good or bad, and let it go. Don't let them do that, let the parts run right up to operating temperatures, as that is where most failures occur, not when cold (you did mention the problem occurs at or near 30 minute operation times).
Do check all grounds, especially at the coil, there are two. First is under the coil, in the form of a buss bar from the coil yoke to cap term GRD, second is the wire out of the coil to one of the coil mounts. HEI in cap coils MUST be grounded. This wire grounds the coil to its yoke, the buss bar from the yoke to the 3 wire connector center terminal on the cap. From the cap, through the terminal, ground continues to the distributor body, with the end of the ground wire being retained to the distributor body with a screw. Any dirty/loose/corroded terminal will increase coil resistance, which can help build coill load, overloading modules to failure.
Check the resistance of the spark plug wires as well, 'rule of thumb' is usually 1,000 ohms per foot of wire length. Wires must be magnetic suppression, spiral, or "magna" core, and 8mm. Resistor plugs are OK, but, critical, spark plug gap should not exceed .045 inch gaps, MAXIMUM.
On the two different coils for large cap HEI, yup, there arte. One is the one to use, fits most applications, .045 plug gaps. The other was a cap that was supposedly to help with emissions control from a larger .060 to .080 plug gap. These large gap coils were used mostly on Olds engines, and, some very few Pontiac, Cadillac left hand rotation distributors, UNTIL it was found the massively giant plug gaps were taking those coils, and HEI modules right out from too much resistance created heat. The factory then issued a FDM to order the effected coils replaced, with the smaller gap coils.
