On the 15-91 the distance from the needle to the pillar is 8 inches. It is probably very close, if not the same size as your 27. You could probably swap the 27 head out of the cabinet and replace it with the 15-91. You wouldn't have to remove the treadle belt from the cabinet, just let it hang, but I don't think the cabinet will close, but to sew and use the cabinet as a work area, not a problem.
The 31-15 is an industrial with a 19 inch long bed. The distance from pillar to needle is about 10 1/4 inches give or take. Most come with a clutch motor if they have the power stand with them. They are a little noisy because a clutch motor is always on. You can swap out the motor for a servo motor which will only use power when you depress the pedal. A power stand takes up much more room than a cabinet that can be closed. My stand is 42 x 18. My top is narrow, they are usually 22 inches deep.
I paid 125.00 for mine off of CL. I posted a Want To Buy ad. It took me days, and I mean daaaays to get it clean enough to sew on. I don't even want to think about what was covering it.