Spain is easy, as it was quite able to become a manufacturing/finance powerhouse. Things were good until, as we know, a series of unfortunate turns made it increasingly difficult to maintain their power. Here are some few points in favor of a longer-lasting superspain:
-Relatively developed Urban centers(Valencia, Barcelona, Toledo, Sevilla, Granada)
-Many Jewish and Moorish craftsman communities scattered around, mostly in the south(before the Inquisition, that is)
-Population(around 10 million by the begginning of the 16th century)
-A wealthy domestic market for it's goods(the Low Countries, assuming they somehow end up inheriting it in this ATL)
-Unimaginable amounts of bullion from the Americas(if colonization goes in a similar fashion to TTL)
Just make the Spanish use those advantages in a more effective way, while also keeping them from fighting half of Europe(screw those habsburgs).
Portugal is trickier, as always, since it's considerably smaller than Spain. However, that didn't prevent the portuguese from punching way above their height and succeeding for a time.
Their age of market exploitation could always be stretched a little longer, and some extra land in Iberia(Galicia, for example) and north Africa is always helpful. Also, keeping them from falling under a personal union would do wonders, as being isolated from the rest of Europe almost always worked in favor of Portugal. Besides that, I can only think of a stronger burgeoisie that takes part in it's overseas ventures, and possibly an earlier discovery of Brazilian gold(that came a bit late in OTL).
I don't know a lot about Italy around that time you're suggesting, so I'm unsure on how to go about it.