So, you plan to shoot film that's six years past date a stop underexposed, rather than a stop over (as would be the usual rule of thumb)? Have you tested a sheet or two to check for age fog? Likely not a big deal at that age with ISO 25 film, but still, you can test, so why not?
Xtol stock (replenished) is my current standard developer. I just processed four rolls of .EDU Ultra (aka Fomapan) 400 since lunchtime. Lots of folks like to use Xtol diluted, too. I have no experience with Perceptol, but I've used a lot of Parodinal (homebrew Rodinal work-alike), and for ISO 25 film at EI 50, I'd dilute 1:50 and look up Push +1 time on the Massive dev chart -- but unless you'll be shooting in the very early or very late daylight, or expect a lot of heavy clouds, I'd really suggest shooting at box speed (after verifying the film doesn't need extra exposure due to age) and giving normal development.
I'd expect Xtol 1+2 or 1+3 to give better shadow retention than Rodinal or derivatives at any dilution, and little if any difference in sharpness.