I have done the power steering refit on my '66, C-10 and am currently doing it on my '66, K-10. My C-10 had the factory stock manual steering box that I upgraded to a power steering setup. I purchased the entire kit, including the box, pump, hoses, brackets, hardware and frame stiffener plates on eBay for $375.00. It is all new equipment. I did it by myself on a Friday evening and Saturday morning.
I am refitting my '66, K-10 from a cobbled up job the previous owner did. I used the REZ Engineering kit that I also purchased on eBay. Both retrofit plates are virtually the same with only subtle differences. They both both work very well and accomplish the correct geometry and box-to-column alignment.
This upgrade is amazingly simple, requires few tools and requires relatively little skill, except for welding the steering column back together. Check eBay for the complete kit or just the adapter kit. I have seen the adapter kits priced from $60.00 to $95.00 plus shipping and tax. I'm not sure the difference in quality or fit, but both of the kits I used worked very well. There is a very good tech. article on Classic Industries website for the power steering upgrade for '60-'66 Chevy trucks. I printed this and used it in conjunction with the instructions included with the kits I purchased. Classic Industries also has a tech. article for the power brake upgrade utilizing your stock single chamber master cylinder and you do not have to bleed the brakes after the install of the power brake booster. Best of luck with the upgrades on your '66.
