Got to thinking. Do you know the tooth count on your transmission output sprocket? I run a 23 and it can get snug in there but if you keep it adjusted it's o.k. This is where an idler helps in case you're not right on top of it all the time. There has been some self clearancing over the years but I figure it's only going to clearance so much and it's done as long as I keep an eye on the chain.
Some guys have a 24T sprocket. I don't know how that would work with an O ring chain.
When you adjust the chain tension have some weight on the bike. I don't have any exhaust on the left side so it's pretty easy to reach down there from the seat and feel the slack.
edit to add:
Shame on me but I skipped right over the fact your bike has been lowered. Yes this can be problematic. I slammed my 76 for a while but brought it back up. Still not stock height now but it was too low, I was always grinding aluminum with the chain and steel in the curves.