You must have bought Paul Hahn’s Lesters. Below are a couple pictures of his early '77 R100RS (40mm exhaust) with the widened rear Lester.
The bottom end on Paul’s engine failed a several years ago. It had ~220,000 miles on it including several seasons of racing (note the orange & white AMA tech inspection sticker on the upper fairing panel above the turn signal in the first picture below).
Those Bridgestone’s he had on the wheels when he was selling them had low miles, but they have to be 7 years old at this point in time - I would change the tires next season after you get the bike sorted out. The rim was widened offset to fit a larger tire in the swing arm, but if I remeber correctly only to 3”. The Battlax’s run small, so if you stick with them, the 140/80-18 is a good size. If you change manufactures, I would go with a 130/80-18 on a 3" rim. Measuring the 140/80-18 Battalax vs. an Avon 130/80-18, both mounted in 3” rims, the Battlax was only a bit wider. On 2 front tires that we measured, a 100/90-19 Battlax was the same width as an Avon 90/90-19. The comment someone made earlier about watching profiles is very important. You want to use a rear tire that has an overall diameter of at least 26” to keep the back end up. Your steering will be noticeably slowed going with a smaller diameter/profile tire in the rear (you are dropping your back end height).