I'm chewing on this exact dilemma myself right now. I've got a nice, ready to go set ov 516's and a set ov 915 cores that need everything. I have a new set ov Manley 1.81 exhaust valves to use. I was just gonna have the machine shop cut the 516's for the big exhaust valves, clean up the ports, cut the guides a bit shorter for a bigger cam and call it a day.

But for an extra $300 i could have the same stuff done to the 915's (sell the 516's to make up some ov the $500 cost).

I'd feel better about porting the 915's as i have never done 516's, and i'm a little wary ov going too far. I've done 906 and 915 heads so far with good results.

So yeah, Hmmmm...