All I can say is I daily drove one K5 I had 65 miles each way to work and back for a good 6 months ,till I found an apartment close to work..then I bought others and split the drive time between them for another 2 years..
I had a '72 K5 that I bought well used for $1800 ,it had some rot on the floors and tub,but not extensive,I patched it up and over the course of driving it I eventually had to replace the u-joints and wheel bearings,nit until I put a good 15,000 on the originals though..
It's biggest "issue" was its TH350 refused to go in forward gears until the engine was warmed up 5 minutes or so..after that you could drive it 500 miles and it'd shift fine,no slippage..but the next morning,back to square one..
The engine was the original 350 ,4 bolt main,the owner replaced the cam with a Crane mild grind for low end torque,that engine always ran strong,and when I first got it,someone had put a 283 intake and the dinky 2 bbl carb on it..
I thought ditching that for a 4 bbl aluminum intake & Carter AFB was an upgrade,but in reality I felt it had more low end pulling power with the 2 bbl..after I put an SM 465 in place of the TH350,(after driving it like it was for almost 2 years with no decline!)--it had to be downshifted into third on hills I formerly climbed in 4th !..
I tried a Q-jet and it was slightly better,but no big improvement..
Had I not sold that 2 bbl setup,I'd have put it back on..it was hilly where I worked and lots of steep ones--with 3:08 gears it needed all the pulling power you could get..it got about 12-14 mpg no matter what carb was on it or how I drove..
Only other thing I did to that engine was put an HEI ignition distributor ,that made it run smoother and then I had no points to deal with,but I drove it a long time with points too, and never had any issues..
I carried a spare ignition module with me,one would croak once in a while for no apparent reason..
I started carrying a spare HEI distributor "just in case" but do not recall ever having to install it on the side of the road..
I had a '75 K5 that was 2WD,with a 250 six and a three speed manual--drove that one to work in the summer mostly,and I put at least 10,000 miles on it before the camshaft gear that was fiber decided to shed its teeth about 5 miles from home on my way home from NH ,a 100+ mile ride..that was the only major malfunction .
It already had over 150,000 when I bought it..
I decided to put a 307 V8 in it I had rather than fix the six,it was not powerful enough and had a lousy 1 bbl that surged and an intergral cylinder head,those sixes were not like the better earlier 250's..
They were just as rugged,but they ran weird and pinged a lot..
I'd say older GM truck are reliable as any other ,and probably better than most of the rest,parts were common & cheap,they were very forgiving,and overall I do not recall getting stranded in any of them,despite the number of hours and miles I put on them...not to say I didn't have to limp home several times,or replace a belt or hose that failed "on the road".but that was more my fault that the truck's for not replacing them sooner..
Maybe I'm just lucky but once I got the carbs dialed in I never had any problems with flooding,etc on the road..we had better gas then too,carbs don't like todays gas so much..
That was all we had back in the 80's to drive..we may not have had overdrive,but we got there and back,and the real beauty of an old truck is YOU can fix it,they are a lot simpler than newer EFI computer controlled vehicles.
There is more room under the hood,the only "disadvantage" in my opinion is they don't have all the safety features as newer vehicles,but since most of the air bags either are defective and on a recall or just fail to deploy half the time on many newer vehicles,maybe thats no big deal..a heavy old truck has the weight advantage over a lighter SUV ...
