I have a 4.250" stroke rb crank with Mopar rod journals. So the question is can I have the rod journals cut to 2.200" Chevy journal and have it stroked to 4.375" According to my math it will work but I could be wrong.

Absolutely! You could even have it offset ground to 4.400 stroke.

Just some on paper math: to gain an eighth inch of stroke (4.375-4.250= 0.125") you'd need the rod journal to be offset an addit'l 1/16th (.0625") so the journal will need to be reduced at least a 1/16th , maybe a hair more. On paper I am GUESSING that it would be doable as you have .175" of journal reduction available (2.375-2.2=0.175") and you need .0625" of that. I'm assuming you have the rod journal width differences between Mopar/chebby taken care of on this. EDIT 4.400-4.250= .150" stroke increase & needs at least .075" journal reduction and you have .175. Hell I'd go 4.5 & 440 source has the pistons for it (using 7.1" rods/.990 pins)

I'm actually going to put it in a low deck. I'm going to have the mains cut down to B size and the counter weights cut down. Stroked to 4.375" use a Chevy 6.535" rod and a flat top piston with a CH of 1.25" and make a 528ci low deck.

You could offset grind it to as much as 4.425, be better to go 4.420 to make sure journals clean up

Maximum possible stroke change = old RJ - new RJ - .005" safety
2.375 - 2.200 - .005 = .170
Maximum new stroke = 4.250 + .170 = 4.420

Your only problem in using that stroke in a 400 block will be the rod to the oil pickup boss. I haven't used that stroke in a 400 block yet, I have done two 4.300 stroke steel rod motors and had to grind around .130 off the boss to clear the #1 rod I do use longer rods with a custom piston also, less cylinder wall side loading with the longer rods, lighter pistons also