Which Kadee to use for MTH Premier Mikado PS-3 conversion.

If you cut the trip pin off, use whatever box you want because the short out risk is low. If you leave the trip pin on the kadee, the plastic box is safer. I use plastic boxes on engines, in case there’s a high spot on the center rail that makes contact or something drops in a switch. There’s no difference in strength as long as you put a screw through the center hole in the kadee box.  The screw takes the load. I doubt you can break the box before the engine breaks traction anyway.  805s will grab lobster claws, the 740s don’t. Make yourself a conversion car (Kadee on one end) and the problem goes away. 

Seems odd that someone doing an electronic upgrade would change out the kadee coupler. Also I wasn't aware Kadee makes plastic couplers only plastic boxes. Some kadee knockoffs are plastic though. 

Pete

Looking at the Kadee web site, the 804 appears to be equivalent to the 805, but all plastic. There's also an 816 (metal coupler with plastic gear box). As suggested on this forum, I'll use the 806 short box on an MTH fixed pilot diesel. The 806 is plastic.

I have at least one pair of every O scale coupler Kadee offers.  I would suggest you get one each also.  That way you can experiment and test fit until you find what works for you.