I can help. If it's a 27" bike, like the 1983 era, pretty wide, probably run 27x1.5's if you didn't want fenders.
roccobike is right. The post-85 Lemans had rather wide box section Araya's, and could probably run 28's, and had decent clearance.
In about 1988, they went to much narrower rims, and the bike had a little less clearance, in my opinion.
I'm building a tourer right now out of a 1987 Lemans RS. I can probably run 25's with fenders, and 28's may be tight.
As far as cyclocross 35's, I don't think so. The best way is to try mounting a wheel.
Friends don't let friends drink and wrench.
1985 Raleigh Competition
Racing USA Series-Coleman made me do it.....
1987 Bridgestone Radac
- Aluminum (sadly, the frame is toast, RD hanger snapped off)
1988 Centurion Dave Scott Ironman Master
1989 Centurion Carbon-R
- Carbon Fiber