Need Help:1967 GMC 4x4 lwb step side

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I am being offered to trade a motorcycle for a 1967 gmc lwb stepside with a 305 V6. I am very interested in the trade, but I have some concerns. The main one being that the tranny(3 on the tree) slips out of 3rd gear while driving. I believe that it is a muncie 318 tranny. I am having trouble finding parts to rebuild this tranny. I would like to keep it original, but I would also consider another transmission if it was an easy swap and still 3 on the tree. Is this a reliable tranny?

I am also wondering how hard it is to add power steering and front disc brakes. The truck has minor seam rust that I am not to concerned about as well as small pinholes in the door rocker. I am not looking to make it a show truck so I can easily clean this stuff up. One odd thing though is that this truck has a large rear window. So I imagine that the cab must have been swapped at some point.

I would appreciate any concerns about this model of truck as well pros. Thanks for any help you can give.