The most economical way takes time and knowledge.
Your best bet is to do an advanced search and you will find several threads on the subject. Putting aside the threads on adapting what people can cobble together I found 2 steering boxes mentioned the most.
The HF54, as mentioned in this thread and several others:
https://www.steelsoldiers.com/showthread.php?170499-HF54-power-steering-for-my-M35A2
And the Saginaw 710 that Waterloo features. Several part suppliers (web search) have kits that seem to range from about $3k to $5500. A note on Waterloo, I recently had problems with them and if you order from them make sure you use a credit card.
The cheapest way I've seen, if you don't already have the parts lying around still involved at least $1k in parts (again, search the threads), months finding those parts and a little luck thrown in.
