SB rockers hitting springs and retainers

The shimming would have been done to get the rocker to wipe more evenly across the valve stem, not to necessarily address any contact with the springs..... well, that is if it was done with any knowledge on the matter! We recently put stock stamped rockers, 273 adjustables, and PRW's on a new Edelbrock head. Both the stamped and the 273 adjustables had some interference to the springs/retainers; it was not on all rockers in either case, just some of them. So I would expect neither to fix you problem with these springs.

As said, the retainers seem to not match the springs.

Answering the question on the valve lift that you have is important. After checking the 3 rocker sets as above, I would be looking at aftermarket roller tipped rockers if going to .500" lift or more; the stock types looked to be at their limit of contact on one edge of the valve stem or the other at that level of lift; remember, the stock systems were designed for lifts in no more than the low-mid .400" range. (Again these valve stem contact checks were on Edelbrock heads; I have not repeated the checks on stock heads.)

I read several very negative experiences about the Speedmasters used in multiple engine brands and decided to stay away from them. I am OK with cheap but adequate, but these do not seem to qualify for that category.

If you grind the 273 rockers, I would do so with the axis of the grinding stone parallel with the rocker shaft hole. That would put the grind lines in an orientation to be less likely to cause any cracks to start; the grind lines will be parallel to the strain in the material.