[Crex]

Couple of things you might want to consider.
The wheel weights are likely to be cast iron, if so, you will find it very interesting welding RxR track to CI. May have to bolt them together. If they are steel you should have no problem.

The RxR track will "sing" pretty loud due to the thin waste. You can minimize the singing and greatly increase the usable mass of the track by finding some drops at a steel supply place cut them to fill the web on both sides then clamp and weld into place (this will in most cases nearly double your weight).

With a little thinking you can use Ed's idea for a hardy hole and incorporate it in with the fitting of the plates.
Also consider what kind of hardies you intend to make/use. If they are the type that the energy used for operation is over the hardie hole you will need to have more mass below point of impact - such as a cut-off, some styles of fullers, bicks and other tools. If they are designed to just key with the hardy hole and rest over the mass of the anvil, that issue is not so important.

Think them through. Have fun with it.