Cleon
Legend
I think the original breath weapon has 5d8, but 5d6 also seems reasonable. These are pretty low CR probably.
I could go either way on the exceptional advancement.
After glancing at post #901 the breath weapon did to 5d8. That's pretty meaty for a 5HD AD&D monster, so 5d10 would be a closer fit with the original.
But if we keep it, I would still like its breath damage to increase with Advancement, but 1d10/HD seems to much to me. Maybe an extra 1d10 per two extra hit dice? Then a 9HD Huge lindorm would do 7d10 and a 15HD Gargantuan one 10d10 with its breath.