In my humble opinion.....
Most guys don't drive their collector cars enough to need lead substitute. I might worry if I was driving 10-15,000 mi a year. Short of that I wouldn't bother.
But,if you have access, and really want lead..... a gallon or so of 100LL avgas would be plenty in a 20 gallon tank.
10-15 thousand miles....lol, I'll be surprised if I put 1000 to 1500 on per year, I'm just gunna keep gassing up at the Chevron down the road with mid grade and not worry about it!