I'm no expert, but would not a waistcoat/vest be standard for a lounge suit of that era?
I don't know if they'll give you much option with MTM, which is still going off of pre-made patterns, but do bring it up. The cut edge of the bottom of the armhole should be 1" from your underarm.
Waistcoats may have been 'standard' back then, but two piece suits were certainly around.