Dave,

Almost all 3/4 ton and up GM trucks had frame rails that are parallel and 34" apart (outside to outside).

Here is a link to some pictures I took when installing an '87 GMC 1-ton (w/Duals) rear end under a '55 2nd series 3/4 ton.

http://rides.webshots.com/photo/2263009200033981030qDaphk

This newer rear end is the same width (61 or 62 inches, I'm travelling and can't check at the moment) as the original. I'm under the impression there are wide and narrow versions of this later rear end so I'd check on that when you looking.

'38 Chevy 1-1/2 ton
'49 Chevy 1/2 ton
'54 Chevy 6400 2 ton
'55.2 GMC 3/4 ton
'56 GMC 1-ton

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