Update on this: I measured the installed (upper) seal as carefully as I could and started hunting for a replacement. Frankly I was soon overwhelmed. I am familiar with seals having been replacing them on various items (mostly automotive) for decades. But now I was faced with trying to engineer one. Most of the seals I deal with have the spring around the ID neck of the seal. Keeps it tight on the shaft and precludes leaking under pressure. So, the fear of a spring type being too tight was a concern. However, in my reading there are seals that do not have a spring to, well, not be as tight. Knowing that the distributor shaft does not see any oil pressure against it, I was thinking I could do with one of those. And then there are different lips, single, double, etc.
I bought a couple of seals I thought would work they but they will not. Recall I have not removed the old seal in fear of destroying it to determine spring, no spring, single or double lip, and exact measurement of its OD.
And during my search I ran across yet another replacement complete distributor. This one for Vortec 5.0, 5.7 etc. but not advertised as marine. It was a plastic not ‘billet aluminum’. And get this: was $43 with free shipping. I am talking a complete cap, rotor, etc. replacement distributor. I have been buying the Sierra 18-5247cap/rotor kits for $75! Are you kidding me? Thus, as Click and Clack would say, “for science’s sake” I got one. Does it have the vents like the OEM making it SAE J1171 compliant? Is it a good quality? Inquiring minds want to know. I’ll post what I find out.