The small Grendel case doesn’t really have enough capacity to feed a 26” barrel. It’s only gaining 15-18fps per inch, so we’re talking 30-35fps difference between a 24” and a 26”. I wouldn’t cut off my nose to spite my face there - I run a 20” Grendel out past 1,000 yards without any complaints. The juice really isn’t worth the squeeze to run a 26” custom barrel unless you’re competing and needed every yard of supersonic range you could get, in which case, the Grendel is a poor choice altogether.

Comparatively speaking, you’ll be money ahead to avoid rebarreling and doing a case-family conversion on your SPS: having a smith open the boltface of a 700, converting to an M16 or Sako style extractor due to the unavailable size in factory parts ($150-250), buying a new barrel ($375), having a smith thread, chamber, and crown or muzzle thread ($250-400), buying a new mag box and follower and having a smith cut lips, block, and fit for proper feeding of the Grendel round ($50 + $150-200)... you’ll easily be into a Grendel converted 700 for $800-1,000, plus the ~$600 I expect you already have into the base rifle (donor action). Alternatively, the CZ 527 American Varmint is available around $700-800 with a 25.6” heavy barrel. The Ruger American Predator with 22” mid weight sporter barrel in 6.5 Grendel only runs about $425-450 street price. The Howa Mini 22” mid weight barrel runs $450ish. They offer a new Model in the MDT Oryx chassis for $1200, which for the money, would be far better featured than a $1400-1600 sunk cost SPS.

I’m always a fan of custom work to make a rifle what you really want, but I wouldn’t shoot myself in the wallet just for 2” of barrel length (or .4”, in the case of the CZ).