A #58 is real good baseline for a stock 1920 on a 1973 SL6. It will get good mileage, however, if you still have a bog with the #58 a #60 or 61-2 should be a noticeable improvement. If this does not solve the problem, then re-check the timing. You should have at least 10 degrees initial not zero degrees. Hopefully your mechanical advance will add another 15 to 20 degrees for at least a 30 degree total by 2500 rpm.
If not, then the weights are bound up, old grease or the springs are too stiff.
That should have a 15R governor in it, (long slots) for lots of mechanical advance.

For reference: I am running 16 degrees initial and 16 degrees mechanical and another 20 degrees with the VC-208 can.

A #64 is too much.......

_________________
Aggressive Ted

http://cid-32f1e50ddb40a03c.photos.live ... %20Swinger


74 Swinger, 9.5 comp 254/.435 lift cam, 904, ram air, electric fans, 2.5" HP2 & FM70 ex, 1920 Holley#56jet, 2.76 8 3/4 Sure-Grip, 26" tires, 25+MPG