backspacing question

[krafty33]

I am trying to decide what backspace I should get for my wheels. I have a 76 K-10 with a 4 inch lift. I plan to put 35x12.5X15 tires on.The backspacing choices I have are 4" and 3.75". I am assuming the ones with the 3.75"would stick out 1/4 inch farther than the other. Are there any fitment issues I will have with the either backspace. I know I will need to trim the fenders to fit the tires

[bigwillie82]

got the same set up with 3.75 bs and the thing is that when you turn left it will eat the fender. and a good dip will rub on the back of the fender, but my leafs were sagging. but new leafs fixed that.