Its a choice...
I can see your point too--sometimes its nice to just roll a more modern chassis with all the newer goodies and power steering,etc,under an older truck..its just a lot more invloved,but either way is a lot of work..if your old frame was rotted or damaged,then it would make more sense to just use a later chassis..and the only way to go of converting a 2wd to a 4x4..I did a few 2wd trucks by using the 2wd frame and a 4x4 parts truck--..I learned its a lot of time going that route!..
Had the 4x4 frame not been junk,it would have been a lot faster and easier to just swap chassis than drill out 50 rivets and swap nearly everything out--there are a LOT of things different on a 4x4 than a 2wd frame--for example,we never thought we'd even have to touch the rear springs--but we learned the hard way BOTH rear spring hangers were different!--after 6 nights of removing the 2wd front axle/crossmember assembly,and installing the Dana 44 straight axle--the truck looked like it was taking a dump!--the rear end was 4" lower than the front!

..time to sharpen the air chisel again,and cut 20 more rivets off each side!

--we had to drill tons of holes,and the bushing holes for the front springs rearmost shackles,etc...then the driveshafts didn't fit either!
It was a 3 week nightmare,and I'd never been so sore in my life after all the hole drilling and rivet busting!..not to mention being under the truck that long on my hands and knees!...the kicker was after all was said and done,the truck never seemed to track straight--it always felt like it was "crab tracking" driving it down the road,compared to my stock 4x4 I had!..It "looked" fine,and we even had it aligned at a shop,and they said it was "off" as far as toe in a bit,but no major misalignment!..good thing it was mostly an off road rig that didn't see city streets to often or too far!..
I had a T-221 Rockwell case in my 72 Chevelle wagon I had that was on a 69 Suburban 4x4 chassis with a TH350 and a 454 from a 73 Impala--it was quite the redneckmobile!...I loved that whine the t-case made--everone I passed swore I had a blower on it!..

..it did get annoying after 50 miles on the highway though!.I still miss that car,I should have kept it,but it was getting pretty rusted,and harder to get an inspection sticker for every year..no one wanted to risk their inspection licence by giving me a sticker despite it being properly built and safe as a normal wagon..so I got rid of it for 1200 bucks!

the 454 was worth that alone..
