If you are thinking of using the W2 long valves (~5.25") I have seen this done by milling the rocker pedestals and using aftermarket shaft blocks to raise the shafts up. The blocks for the x head were drilled in center while the w2 race head blocks are drilled off center. I have not seen these on center blocks for sale any time lately. I am sure there are other creative solutions as well.
If it were me I would not modify the x heads for long w2 valves if you intend to upgrade later. The cost could be comparable to other.good choices in aftermarket heads. You could run what you have with a good valve job and minor port clean up and save foe a.later upgrade.
Look at 'Special Notes:' on this page........."Rockers are designed for big lift cams requiring .100-.150 taller valves."
http://www.harlandsharp.com/sb_mopar.htm
Mostly I'm worried about the cam which is why the long valves. I considered w2 long valves but there's a lot of valve train geometry issues in a std head.I was thinking about a .100 long valve with 1.6 rockers. It depends on where the change in the ratio is on the rocker though I think. May just save up for the heads anyway but not go w2. I don't think I can fit the cam I want with the current X heads I have on the engine now. (it's at .507 and the retainers are pretty close but changing retainers may help with clearance issues.) Just thinking outside the box.