I'm new to welding, and this forum - apologies in advance for basic questions.

I would like to build a dolly for a car shell body (not a road/highway dolly) - but basically a jig with 4 castors that can roll a unibody shell around the garage.

Ideally, I would like to make the dolly's length and width adjustable, by having tube steel of 3"x 3" x 1/4" slide *over* 2 1/2" x 2 1/2" x 1/4" . On paper, a 2 1/2" square tube (1/4" wall) might slide inside a 3" square tube (1/4" wall). I just tried to place a steel order, and asked if "...a 2 1/2" tube would slide inside a 3 " tube ? - and was told it wouldn't.

Is there a good source for accurate steel dimensions? Or other adjustability suggestions? Open to someone else's plans as well. Any / all comments welcome.