The RDG toolholders will not fit the Boxford toolpost. But they are very close to doing so. The dimensions of the angled faces are such that they will mate correctly, but the problem in fitting as expected is caused by two differences: the dimensions of the clamping flange, which engages in the tee slot of the holders, and the amount of eccentricity of the rotating cam operated by the socket spanner. Boxford make theirs to tighter tolerances and their eccentricity is about 0,5mm, giving rise to greater clamping for the same applied torque on the socket spanner. The RDG ones have about 1mm of eccentricity, enabling slacker tolerances and hence cheaper manufacture.
It is possible to modify the Boxford toolpost to accept RDG toolholders by grinding a bit more off the cam to give about 1mm of eccentricity, coupled with removing a small amount of material off the mating face of the clamping flanges if required. Note that these parts are hardened and once you've taken the material off, it's not too easy to put it back on.
Probably your best bet is to sell the Rapid toolpost and any holders and buy a complete RDTG set (Usual disclaimer). I kept mine and adapted it as above for use on my Myford Super 7 – so I could use the same tools for each lathe. I wrote an article in ME about it a little while ago.