Yes, to recap. It's a 66 Mustang coupe, with a non-original 289/302 and C6 auto. The C6 took a dump, and the stang originally had a manual in it. Pulled the C6 and dropped in a 4-speed toploader from a friend's circle track parts car. Taking another look at the casting numbers comes up with:
D9OE-8015-E3A
This decodes out to a 79 Ford LTD II car 5.0L (302) V8. Since this engine was mated to a C6 auto, it had a flexplate with a large counterweight welded near the ring gear for balance. To do the manual tranny swap, We installed a new clutch, disc, pressure plate and piolt bearing along with the flywheel, all for a 67 Mustang. tihs was just taking a stab at what motor was in the mustang.
I think this is a fairly common problem when swapping from an auto to a manual, due to the type of crankshaft in the motor. While all of the Ford smallblock V8's pre-79 use a 28 oz imbalance flywheel if they're not internal balanced, a 50 oz flywheel was used with 80 up motors mated to a manual tranny. So, the only two options we have are to try a newer 50 oz flywheel, or bolt on a neutral-balance one.
That's where we're at for the moment, trying to locate a 157-tooth ring gear, 50 oz imbalance flywheel for a 80-up 302 V8. JSM84