340 Heads Recommendations- Aluminum vs Cast J

Need advice on where to go with my head situation in the Duster, been looking at posts on here Speedmaster with work or just rework the J heads and call it good, I’m just gonna be street driving and maybe a rip down the track once ina blue moon for fun, no racing.

Trying to decide money vs gain on either building up bare speedmasters or reworking my J heads, would use valve springs for my cam.

340 supposedly .30 over
915 J heads- unmolested
Crane cam 284 H12 pt# 150062
Eddy Air Gap intake
4spd w/ 3.55 suregrip
LT headers 1 5/8
Holley 4150 dbl pump
Supposedly steel crank, haven’t been able to confirm if that’s true per previous owner

Anyways, suggestions, advice, feedback welcomed, thanks dudes!

B562583C-EEA7-4B6D-B9E8-8F49296F1C32.jpeg