4l headswap valve spring questions

0.500" is max, but I'd go a little less than that, like 0.490". The Comp Xtreme cams have too much lift for the stock springs IIRC, and the Comp recommended springs are really for high rpm racing -- I consider them way too strong for street use. You will need off-road oil or a high pressure additive to run them also. Anything much over 300# per inch on valve springs is too much for regular oil. I think it's Crower that has springs around 312#, those are the strongest I'd recommend. Mopar Performance sells a performance spring for the 4.0L that is really just one of the V-8 springs. It's not a lot stronger than stock 4.0L springs, but is stronger... I forget the rating though.  That's what I always recommend, and something like the Isky 256 Supercam (mild) or 262H (a bit more aggressive but still not bad, and works with all stock parts -- works with stock gearing, but would "like" a little more gear, like the optional ratios, 3.08 or better).