I have a 70 Chally with 8.75, 3.90:1, 340/ 727 and a 71 Cuda, Hemi, 5 speed (T600), 8.75, 3.23:1 ratio that I want to switch around. Would you swap the entire rear ends or just the chunk, which is easier? They are both in great working order.

Probably just the chunk.
Otherwise you're dealing with leaf springs, brake lines, etc.

Are they both large yoke, both small yoke, or is one large and the other small??
Makes a difference with u joint fitment.

I'd do the whole axle, just me I dont care to deal with the gear oil mess/end play etc. it'd be heavier lifting but easier in the long run when done. Just have to bleed out the brakes. As said check that both are 7260 or 7290 yokes

I'd swap the pumpkins.

You going to get back with me about the relays kits and wiring?

Just change the chucks out good way to change fluid which should be done every few years anyway If ypou run into Ujoint issues get the 7260-7290 adapter u joint.

Cant I just swap out pinion yokes?

742/742 yes I believe.