What engine mounts are you going to use? The ones that came with the plane?

Yes you can do as you said to hold the weights on. Remember though that lead is pretty soft, so you may want to provide at least two bolt points and use large washers such as fender washers to retain the metal.

BTW: I don't know how you are going to heat lead in a tin box.

The melting point of tin is 449 degrees F, while the melting point of lead is 621 degrees.

There is a company that sells a product called "Liquid Gravity" for RC'ers. It is nothing but fine lead shot. You mix it with epoxy to apply it to any surface, nook or cranny.

You could put the tire weights in a wooden box and affix that to the firewall or engine mount, or you could do the same and put it behind the firewall in the compartment.

If you check the photo I posted above, you'll see that my engines are pretty far out front. I doubt your engine is lighter than my DLE 20's.

Don't forget that you can turn the needed weight into "useful" weight by installing BIG battery packs in the compartment and on the firewall.