Let me give you my opinion.
First of all, with another $4500, your truck could be quite far along. (Like a SOLID cab, all your rust repaired (not painted), and a good running engine[which you may already have]). You know what you got, and you know what you need.
Second, chances are, you're going to lose some of the money that you put into your dearest '59.
Third, as they always say, buy the best truck you can with the money you got and work from there.
Fourth, after you buy this nice little '52. You're guarnteed to have to put money into it.
Fifth, time. You may not have your '59 done for a couple more years (one at least). With this '52, if it is as good as you say, you may be driving it in a few months.
So what I say is if you can get maybe $2500 for your '59. Your not doing to bad and then if you buy this '52, you've got something solid and potentially drivable (slow lane of course). Tough decision, we've all had these times. I say stick with the '59. But we want pictures, maybe I'm wrong.
Good Luck.
Colin