I've always been leary of much grinding on the rockers to clear the springs/retainers. I've seen too many ductile iron and aluminum rockers that had broken there because of it.

It was many years ago, but I had lot of issues like that too. Blocks could relocate the shafts & rockers away, but also changed the height. Certain offsets/heights were designed to work with certain valve stem heights. But then you also need offset holes in the shafts.

I did use 5/16" pushrods, but had to open up the holes a bit in some cases. Thicker walls are an option for strength.

Are there any beehive springs (for clearance) that would fall into the specs you need? Comp Cams and Pac have some that can get over 400 lbs open. But if it's a flat tappet, you may not need that much. It concerns me over 350 with a flat tappet anyway unless expensive alternative parts are used.

FWIW, I had several W-2 360's with cams between .575" and .590" lift. I don't remember if it was an isolated incident related to a particular cam, but I found that it ran just as well shifting at 6,800 rpm as it did at 7,200. So I settled in at 6,800. It may have been with a set of heads ported with the Mopar Performance template kit. 10.22 @ 130 + in a 2,800 lb Challenger footbraking.