unless you want a slight increase in power and do it yourself there is no reason to modify stock heads "period" (like Obama said about health plans, "period" that liar) but seriously the worst aftermarket head is much better than anything that came stock, I have been building SBF's for over 30 years, even those cheap procomps which are a copy of a Victor head will blow away the best ported stock cast iron head, they have some cheap parts in them but you can make them very good. I put a bunch of money in a set or C90E heads once they were good, chevy valves, fully ported, screw in studs, guide plates cost me about 850.00, and then I bought a set of Twisted wedge heads for like 900.00 way back when in 1995 or so and even with a compression drop they smoked what I had, Today I have the same heads but had them CNC ported to 205cc on my 408 stroker, if you are looking for real performance then aftermarket heads are the only choice. I dont care what you do to a stock head, it will not even compare to an aftermarket aluminum head, even a cheap one. I would probably spend a couple,few hundred more on a set of American made heads. Otherwise if you want to go the procomps buy the bare castings and put good stuff in them and grind off some of the casting flash, some get a good set and no problems but its not worth the chance, if its a mild 289-302 maybe look for a set of gt-40x heads new or used, factory quality, not as good as AFR or TW but cheaper. You will find people telling you the procomp heads are barely as good as stock which is complete BS but the reliabilty factor is true, its hit or miss with them, with procomp anything that is the case.
As I have written elsewhere on this forum. the crate motor I bought from Tuff Dawg has the Pro Comp heads on it and God knows what else.
From my conversations with them, they do use some good parts......*when available* ( I can just them rooting around the shelves what's to see what's there ). I am going to ask them for a build sheet when it is done, but.......
At any rate, it's a Dufur, OR. (as in "do for now") motor . I'll break it in right then run the hell out of it till the warranty expires (next fall) then, if it lasts, it's Katie bar the door. I still have all my porting tools. Tear it down & any crap will get replaced. I just hoping they do a decent job on the block.
As for head related failures, who knows? It is unquantifiable. Any bozo can eff up a motor during break in (or later) and one hears more tales of that than are shop related screw ups......it depends of who you're talking to. I feel confident I could serve up a valve seat...& maybe more during break in... if I put my mind to it.
BTW, I use Brad Penn 30 wt, non detergent oil for break in.....it has served me well.