yes that is a problem i found interesting as the Fox catalogues list Fox air shocks for 1980 RM250/400. I have the same problem on my 1980 PE250 which uses same air box and frame as the RM250. What you can do is a combination of things.
1- heat the air box to make it soft to deform and press in in a little
2- fit some spacers between the airbox and the frame tabs which will pull the airbox in narrower. I have tried this and it does work
3- space the shocks as far to the outside of the mounts as possible, with spacers on the inside of the mounts. If you go too far then they rub on the yellow sidecovers though.
4- get new billet top and bottom caps made with a slight offset so the shock is offset out wider. That is the most expensive option but that is what im getting done as i need custom length top and bottom caps anyway to get the shocks how i want them as i dont want to cut the gussets on my swingarm so the lower mount doesnt hit them, but you dont have the worry about that on the RM as they have the gussets cut out from the factory.
I would look for a PE250/400 airbox which will be much easier to find. try asking on the vinduro forums
