It depends on how bad your rust is. If you have severe rust holes in the inner cowl, floor board, toe board, rocker panels, hinge pillars, and front cab supports, I would do it on the frame. Make sure to have rebuilt rear cab mount shackles, and new front cab mount pads, and adjust the cab to the frame per the Manual specs.

1952 5-window - return to "as built" condition | 1950 3100 with a 235 and a T-5 transmission