In most engines, like GMs and others you can buy various length push rods depending on what you ahve doen to the heads. Sadly the Rover v8/Buick 215 is not a very popular engine so no one makes custom length push rods for the masses. The cure is to use shims under the valve bases to again achieve the correct length for the push rod. If you shave the heads and don't make up that distance in the head gasket then you have to somehow correct for the now shortened distance from point A to point B (push rod length). The shims allow you to do this without the need to get costly custom push rods made. The need for it depends on what was done to the heads. If everything is factory fresh and not rebuilt then you should not need them, but if the heads are rebuilt, or the block deck is shaved, it is worth checking as incorrect push rod length means valves not working the way they were designed to. Its a picky thing to check and most don't do it. It really depends on how correct you want everything set up, and what was done to your block and heads.
I do not check PMs. Call or email if you need something.
ECR is not a parts source. We are not in the mail order business.