Ok, take the cover off and take a look at your flywheel. It should look like this428 Cobra Jet Flywheel Identification

 

OK, working on that next...

 

Glo, never mind the flywheel, it doesn't mean anything, and doesn't matter. ALL of the FE flywheels fit, and all of them can be rebalanced on a spin machine in about 10 minutes to work with the 410-428 cranks. If you have the 4 inch stroke ( 3.98 ) then someone swapped in another crank before you got the truck. Bonus for you. The 410 and 428s have been out there since '66, and wheredo you think their cranks end up if the block breaks or something ? lucky guys put them into 390s and 360s. If the engine runs smooth, someone rebalanced the flywheel or used a 410/428 'wheel. Most likely the original 390 wheel was rebalanced, because most of the 410s and 428s came in front of auto trannies. DF

 

Good point. Very probable it was a rebalanced 390 flywheel. If it's not vibrating then its fine. Enjoy it.

 

That is a good point. I do have great power, enough to smoke the 37" tires, but that isnt saying much.

Thanks!