I assume his is 3/4 top and bottom? That is correct for my stock 1960 4x4. If yours is smaller and is a 3/4 Ton truck, it's probably not stock. Does not matter. We constantly fix small problems like this on here all the time. What does the top bolting point look like?
Mine fits up inside a "C" channel cross member and bolt goes thru Channel and shock. This may be a simple matter of finding you a sleeve, which fits the upper shock sleeve/bushing and transforms 3/4 to 9/16. Not a big deal. Supply the dimension from bottom bolting point to top so I can check my truck and see if it's the same as yours. It's possible this shock will fit in all ways except top bolt. It is, in fact, correct for 1960 1/2 and 3/4 ton stock Chevy trucks. Be sure the top bolt is the correct bolt that looks like it fits there, or maybe it's a permanent stud? Pictures would be super.
Something along these lines may work. One of these will make two.
https://www.cachassisworks.com/p-26...for-poly-bushing-yellow-zinc-plated.aspx

If it turns out that this shock will not work because of wrong extension and compression dimensions, I will give you the procedure for measuring your truck (it's easy) and then we will find a shock that meets the extension and compression dimension for your situation.

Please let me know if you are at the point of no longer interested in this issue. Not a problem.

Right is right, even if everyone is against it, and wrong is wrong, even if everyone is for it. - William Penn