[mr48chev]

If you measure from the frame to the floor on all four corners do the front measurements side to side match and do the rear measurements side to match?

Is the measurement from the front of the box to the style line on the cab the same on each side?
A lot of these trucks that still run the stock suspension lean to the left simply because they only had one person in them for years on end and many many miles. A lot of others have frames that are tweaked because the previous owners hauled super heavy loads in them and twisted the frames or they sat for years in a manner that twisted the frame into a permanent set.

I cut the first frame I built for the 48 up because I had welded in a twist in the rear of the frame when I did extensive suspension mods. It looked a lot like yours from the back in that it was off on one side.

Yours won't be that drastic but you need to figure just where the lopsidedness is coming from before you can attack it. You may be able to do something with the suspension such as swap springs side to side or you may have to take it to a frame shop or you might just have to put a new bed mounting kit in along with a shim or two.