Besides the piston rings on a 1988 7.3 being smaller then the latter 7.3's what else if anything was different between the two. In case you didn't know the top and bottom rings are smaller on a 88 7.3 by about .020 and there's only one set left and its in Texas for $200.

 

I ran into this too when i built this 88 for my 92, wasnt much of an issue as im running psd rods and turbo pistons (which are pretty scarce now too). The 88 top ring also isnt a keystone style as the 89 and later iirc, other than that, i think a 7.3s are the same until you get to 93/94.

Im thinking you can use .030" over bore rings for a 390 FE, at least the compression rings. Dont quote me, im pretty sure thats what justin orders for the total seals, and 390 rings can be had in thinner varieties. At any rate might be worth looking into, or just getting std. 7.3 pistons.

Would be interested what you find out, i have a set of good rods and pistons out of my 88, not much use if you cant find rings, but these are probably good enough to go again if youre not picky.