Probably going to depend on the exact block and sonic numbers... but if I had to guess I would think on average they could handle another 100hp. Of course the motors I build generally don't come close to the limit so it still boils down to roller cam for dirt cheap or possibly a little more bore. Right now I am building the early 360 block at 4.030 bore with a .590 solid flat tappet, w2 heads super light rotating parts and stock stroke.

My own personal motor is going in a 97 5.9 magnum block with full ported R/T heads, big hydraulic roller, and at least 410 cubes unless I can sell that kit, in that case I am going to get a BPE 4.25 stroker kit