is this possible? are there any differences in chassis rails between models? will it just bolt on?

I'm pretty sure that the early bays (68-72)are different from the later ones
marcel

Marcel is right they are very different,
easy to see when you have both side by side,

Can't answer specifically, but what I do know is the cover plate (under the front floor) from a '71 fit's on my '77.
Also from other posts about front beam swaps (do a search) it sounds like the beams are interchangable also.
From that I gather the chasis rails are the same distance appart.
Hope that helps.