Help me decide tire size

I thought this was the best spot for this question since I will be doing a SAS. I have the 4" trail gear front rings and 63" chevys for the rear, I will most likely add a leaf and longer shackles to level it out. Ive been seeing a lot of folks using the 3" TG springs and clearing 37"s will not issue. Im in a bind between choosing 37" or 40". They are pretty much the same price for the tires I've been looking at. Thanks in advance.

 

If you just want a cool looking truck to drive to the mall and pick up bro-hoes from In-N-Out put 40's on it. If you actually want to wheel and not break shit, put 37's on it. Unless you have 1 tons, in which case you should know what size tires to go with