a different cab?

[smiley68]

Hello,
I am new to the rebuilding stuff and relatively new to the truck scene. I have a different cab for my 68 chevy and I'm wondering what the legality issues are to putting the replacement cab on my existing truck. Will I have to have a new title made with seperate numbers? (Vin # and Frame #)
The reason I'm asking is I have seen sever cabs for sale that state clear title or salvage title or vehicles for sale that say the #'s match. I'm kind of lost on the matter.

Thanks,
Smiley