Since the 1340 was originally cert on 80/87, and operated several decades on that fuel, power output (limited by preignition/detonation) with mogas should not be affected.
And if you're not operating an aerial mapping ship at FL200, vapor lock should not be an issue either. If tankage is below the carb and a fuel pump is used, that shouldn't be an issue anyway.
But check with operators with more experience - they'll have the right gen.