Best next upgrade?

I'd lean towards conversion to BBP with factory-based front discs and 8 1/4" rear and some wider 15" wheels. O.P. mentioned he isn't trying to get into serious racing it's mostly a fun cruiser. Also put in some ~1.03" torsion bars while you're in there, you'll be glad you did. Car will be so much more enjoyable and easy to drive (and safer!) with modern tires at modern speeds.

With those upgrades the car will be able to put down more power reliably and handle higher speeds when you go for power upgrades. Which I would say for that, longtube headers and a better cam for sure.