How interchangeable are 318 harmonic balancers?

I am (was!) buttoning-up the engine this weekend in preparation for putting it back in the car when I found two of these:
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Left rear, and right front exhaust studs broken off about 1/8" inside the head. They're fixed now, but...that was fun! (insert sarcasm here)

Anyway, back to the harmonic balancer issue.

I got the TDC/piston stop tool out again to check "indicated" TDC (so much easier to do with the the engine out of the car!), and each time I came up with the same answer/result. On the last attempt I ran out of tape (thus it became my last attempt), so bear with me on this picture.

The heavy pencil line to the left (or top) is the correct "before" point, and while it's hard to see here, there is another pencil line right at the end of the tape for the "after" point.
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Needless to say the TDC line on the balancer is not centered between the two, and according to the timing marks on the cover (when I "center it up"), it looks like it's about 6 degrees off.

If you will recall the owner, Bazza, started taking the engine apart because it was running like sh** and overheating. Then he got sick and ended-up in the hospital. Between this, the stretched timing chain, an original distributor with worn-out bushings, and what I'm sure are more than a few vacuum leaks (power brake booster line was REALLY bad), I think I'm finally making some progress toward getting this running again.

I'll leave it up to Bazza whether to get the OEM balancer rebuilt or not, but I need to get it running , so I'm off to buy a new balancer.

Thanks, everyone!

Jim