Not sure if you can shoot a lighter bullet or not effectively. I bought just about every kind of bullet/mold applicable to my .308 Texan and the best overall is a little 115 gr. from Accurate Molds. A close second is the NOE 150 gr. boat tail. Granted, the gun and barrel are both different, but don't be afraid to 'go low'. For $#its and giggles you could buy a box from a supplier to see if they even like your gun. As a previous poster said, sometimes things 'just work' despite what the engineers and experts say from their desktop shooting sessions. That big bullet just seems really heavy and the speed a bit slow, but obviously the previous owner new what he was doing. Just for reference, I'm getting about 920 fps with a 115 gr. from my stock-internals Texan.
Beautiful gun. Good luck and happy shooting.