You have to ask yourself a couple of questions .
1) can you get a title for the good cab you have ?
2) do you have a welder and the metal working tools your going to need ?
3) do you have the dry space to take on a major project ?
4) what's your time frame ?
5) What is your real budget ?
Have you looked in your area what a NICE 69 longbed is selling for ? I've been rebuilding muscle cars and trucks for myself and customers since the 1970's and there has never been a time it has been as expensive to rebuild a classic as today .
I'm not trying to talk you out of it just wondering if you've considered everything that's involved ?
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1967 Factory short bed - Old school
'71 - 350 / 4bolt / 487 heads / Edelbrock C3BX
Muncie M-22 4 speed / Hurst Comp plus
Factory 12 bolt posi 3.73 / 255-70-15
Smoothed firewall / Factory cowl induction
Power disc brakes / power steering / 3.5-5" drop