Aluminum inevitably bends and is hard to straighten. Carbon stays straight as does good (not all) wood.
600 may be too soft with a 31" draw, depending on release and question of Dacron Vs whatever (some say Dacron makes more sense with old bows, some say Dacron never makes sense). I'd guess 500.
I use Dacron on my 60# Groves (on the deceased bowyer's advice) and have just put Dacron on my new/vintage 32# Wing Gull... but I use 450+ on my various longbows.