Lol modify them off! That line brought a smile. In Your opinion is there value in modifying the X Heads? Or is that the performance increase not justify the cost of working them?
You can modify them if want and can have the flow pretty well but you'll get more out of a set of aftermarket heads.
On my 340 I had my x heads fully rebuilt ready to go but I switched to a roller cam through the build so I swapped to a set of Edelbrock aluminum heads. This raised my compression a bit and had a true 63 cc chamber where has my x heads (one was a 73 the other a 76) which were suppose to be 72cc. Mine came of a barracuda that got t boned with ~22000 in 1970 the motor sat untouched till the guy I got the heads from rebuilt that 340 with w2. What I'm getting at is that you'll get more outta that aftermarket heads vs factory.
Personally it's not worth the money to fully rebuild a factory head than port when an aftermarket heads will get me more bang for the bucks.
That said the only time I can see using factory heads is for a resto and even than its roller cam/lifters/rockers and getting them to flow.