IMRA runs rules very similar to the Montpelier Midget Series, with a few exceptions; one of the exceptions is a rule limiting left rear wheels on both Chain and Shaft-driven cars to 8", but with a maximum tire tread width of 10" (MMS has no LR rule). Right rear rules are similar to ours; 10" max wheel width for Shafts, 12" max for Chains, 12" tread for both.
Not sure why you'd run a 10" tire on an 8" rim if you didn't have to, other than for more stagger choices as has already been discussed; how much more rollout do you get on the narrower rim?