Well I'm glad to read you view on them. I'm not alone here. And yes they are some very nice models and the Big Blow pulls a 70 car hopper train around the layout and up 1.34% all day long but only when the couplers hold out.
Must find a MT that fits it.
Big Blow MT conversion is the easiest one there is. It is even mentioned in the booklet enclosed in the box, bottom of page 9. But I used the MTL springs. Just remove the screw holding the coupler box, pull out the box, remove lid, and install the MTL.

I used 1015/1016-B (brown) couplers. New part# 001 94 014. Short shank. You might have to cut the top of the trip pin flush with the coupler body (if it interferes with the gray plastic uncoupler lever (directly over the coupler).

When the coupler box was out of the model, I installed the MTL coupler innards, then I heat welded the lid onto the box using a tip of my soldering iron. You can see the welds in this photo. Makes it easier to reinstall the coupler (without needing 3 hands to keep everything in place).

Here are discarded Scale Trains couplers along with Scale Trains springs they recommended for the MTL conversion. These springs are too long and too stiff to work properly. MTL springs work better.