[86LT1C10]

Now to explain these pics:
#1 The only modification I had to do to the front of the frame to make the body sit down on it. This is at the radiator support area. I also removed the crossmember that sits below the fan. I never got to fabricating a replacement crossmember.
#2 I had to cut a little off of the inner fender to clear the upper A arms.
#3 I used the 80's steering column mated to a 80's Monte Carlo lower shaft to hook to the gear box.
#4 I ground off the 80's cab mounts and bolted the original 58's cab mounts to the frame. You can also see the two frames mated together.
#5 I boxed in the inner side of the frame channel.

I don't see why a complete frame couldn't be used since the stepside beds between the years are virtually identical in how they mount. At the very least, nothing a little modification can't fix. The only reason I did't try it myself is the front clip was from a longbed.

All in all, I would go for the Square Body style. the frames are tougher than the 90's. The reason the 90's are smoother riding is because GM was trying to catch the female market and women wanted that Cadillac ride. When doing this, they lost the toughness of the suspension and frame. That's why in the 90's Ford kicked GM's butt in commercial sales and twin I beam ruled for once.