Substituting an earlier fork for a Showa is a downgrade in my opinion. Showas replaced the other models in '91. Their internals are more robust and slightly less complex. Where did you buy the springs, who is manufacturer and what is the model number? We could get to the bottom of the difference using that information. Before the Showa started being used, the Sport and the Brembo were most often used. Sports have an
S stamped in their filler caps, Brembos have
Brembo stamped on the slider. F&S were another fork—the slider seal seems similar—and all fork types have been swapped among models whenever necessity has ruled. On the old style fork you're using as the replacement, the seal was driven into the slider with an arbor until it was flush. Other models inserted a circlip or spring retainer over the seal, likely to hold it into position and reduce leakage.
When laid out next to the original assembly, the overall length of the assembly with the new springs should be the same. Adjustments can be made by cutting the top spacer but doing so will affect the action of the suspension. What should not be done, as you have considered, is compressing the new assembly into the same space if it is longer. Perhaps springs could be cut to similar length as the originals. That takes tools and experience, or else grit and a dominating sense of adventure. My sense of adventure is focused on the road.
I'd stick with the original springs of the replacement but then the reason to use that old-style fork is lost. If all else is equal, I'd write off the loss or sell the new springs and replacement fork on eBay (once I verified exactly what model the springs fit) and get the correct parts for the Showa if it isn't corroded or otherwise flawed.
