They do not quite fit. See here:
http://www.truckinamerica.com/toppers/BedSize.htm
The width changed a hair... but the beds (short and long) were lengthened by 2 inches in 1993.
That's a very good link (I bookmarked it for future reference). But, it actually says the LENGTH increased a scoosh and the WIDTH increased two inches.
As noted above, the inner bed stampings stayed the same. The bedsides were made fatter so the fender width covered the widened rear axle. Not exactly sure where the length change happened - tailgate area maybe.
Anyway, the beds themselves can be bolted over - i put a bed from a 94 on my 91. Turned it upside down and put 2x2s in the stakeholes in my old box to haul it home - that's how I know they line up. Body lines look a bit funny but to me it was worth it to get a non-rusted out fenderwell area and a tailgate latch with an access panel.
Back to the OP's question. Yeah, I pretty much guarantee the newer cap wil sit on the older box. But it may hang out over the sides and you may have to jiggle things a bit to get the ends to seal up right.
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