The standard Monte rear is narrow enough that a wheel that covered the backing plate is likely to allow the tire to touch the bed. However, you really need to choose your tires and think through how they will fit in the wheel wells before choosing wheels, and only then would you be able to decide how much spacer it would really take.

I am not a fan of spacers because of the way they add leverage working against the bearings. I'd work to figure out the ideal width of the rear before using spacers.

I don't recall the width of the Monte rear but it is pretty narrow. I have a '73 Chevelle rear under my Suburban, which is about 62 inches wide. That is close to the stock measure and works real well under my truck. The Monte is closer to 58 or 56 inches and with most wheels is going to crowd the inner fender.

1955 1st GMC Suburban | 1954 GMC 250 trailer puller project | 1954 GMC 250 Hydra-Matic | 1954 Chevy 3100 . 1947 Chevy COE | and more...
It's true. I really don't do anything but browse the Internet looking for trouble...