I had Vintage Air in my '70 K10 for several years, and it worked well for cooling. It didn't have decent heat, recirculate only was terrible for humid conditions, especially a rain storm in summer. I wasn't wild about the hose connections being inside the cab, under the cover. Rather difficult to work on.
I removed it and installed a modified factory system, with an evaporator core from an '85 or so truck. I had to get custom hoses made to do this though, but glad that I did it. The heat output is great, there is airflow whenever the truck is moving since it has fresh air option. I haven't made recirculate work yet, possibly won't. The blower is pretty quiet since it's under the hood.
Some guys have tried Old Air and liked it, while some swear by Vintage Air.
If you only drive it in the summer, and don't drive it in the rain, aftermarket stuff may be fine. I have truly used my truck in all four seasons, in temperatures from single digits to over 100*, rain, sun and snow.