What kind is in there now? Press fit studs? If yes, you'll have to get them drilled and tapped for the threads for the rocker screw in studs. If they're positive stop studs, you'll have to replace them.
For size, that's up to you, deciding between 3/8 to 7/16. And for the ratio, that depends on the cam you put in.
The reason you change from press fit studs to threaded screw in type is because high performance valve spring pressures will pull the stud out. Has not much to do with the rocker type. If you're using the stock springs, there's no real need to use screw in studs.
If they are positive stop studs, you can sometimes still use a roller rocker. Often the fulcrum is thick enough to not hit the stops. Or you can use a quality washer as a spacer if needed.
But, threaded ARP studs are clearly the best. If you have the money, and the heads are off, it's probably a good idea. But maybe not needed, strictly speaking.
I think it was comp cams (or maybe crane) who use to make a really nice rocker. It was a stamped steel rail type with a stock type fulcrum and a roller tip. Better quality and consistancy, but lower cost. Perfect for the close to stock build.
.boB is exactly right about the press-in studs pulling out. This is, in part, what killed my engine (the other part was me being 18 and stupid...but that's for another thread)! Don't let anyone tell you they won't pull out, they WILL if you have anything more than mildly over stock and plan to romp on it. A couple of mine pulled out so far that you could remove the push rods without loosening the rocker arms (pics on the MySpace in my signature). You don't necessarily have to replace the heads but upgrade the studs and get push rod guides just to be safe (they're cheap piece of mind).
Thanks for the suggestions,
I found and ordered a comp cams adjusting kit for the positive stop guides. At least now I will be able to adjust without the hassle of changing the rod size. Understanding that I need to go to screw in studs if changing from stock spring rates, if I leave the springs the way they are for now, what roller would be reccomended to work with what I have.
Stock heads with the adjustment kit, stock hyd cam and lifters.
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