I can't match the "121" - but the "8578" matches up with the Small-diameter ProBillet distributor. I don't normally post guesses but I would say yes. This distributor is smaller diameter than stock and the stock one fits although you can only get about 120 degrees of adjustment on it once it is installed. Have you heard otherwise?
if im not mistaken, you have to order a special MSD dist to work with a Vic Jr on a 351W. MSD only makes one and I dont think any 'swapping' can be done. Give them a call...or call summitracing.
I called msd they said it would go in but it couldn't be turned.The guy that has it sayes he used it with a Victor JR. Guess I'll skip it. Any body have one that will work for sale.
I am running the DUI Street/Strip Ford with my Victor Jr. on the 351W. Even though the top end is huge, there are no tuning/clearance issues, except I wanted to run the TCP cross bar and couldn't.