I forget what must be done to make this work so time to call upon the vast knowledge of bike forums. I have a shimano ultegra SL compact crankset with chris king BB. The frame it is on is a cross bike with a 68mm shell. I am considering going to a karate monkey which has a 73mm shell, I know I will need to go smaller on the big chain ring since the km will only take a 48 tooth ring and I have a 50 but what else must I do to make it work if possible? Thanks in advance

ultegra SL compact won't fit on a 73mm shell, so you'll have to face the shell down to 68mm.
I'm not even sure a 34T will clear the chainstay, because shimano's outboard BB double cranksets have a very narrow chainline that's not adjustable.

leave the road crank on the other bike.

External Bearing? means new crankset then, Just Buy the MTB set,
and the chainring you want to use, instead of stock

cutting down the shell? all 5mm would have to come off the left side.

That crank has the spindle built in so you can't fit it with a longer one. You could face the bottom bracket shell down to 68 mm but that would be a lot of metal removal and, as noted, your chainrings probably wouldn't clear after you got finished. Two possibilities:

1. Get a suitable frame for the current crank
2. Get a suitable crank for the current frame.

as a side note, the road triple cranks from shimano sit out further and use about 9mm of spacers for a 68mm shell, so it might be possible to use that on a karate monkey.