The heads with the 40 taper spindle will not fit a BP style clone the ones with a 30 taper will. The 40 taper heads are 5 hp and fit a larger pattern mill. BP made a 30 taper series 1 size mill they are not very common but a 30 taper spindle can be installed in a standard head.
I'd be pretty skeptical of a 40 taper on the same chassis as a R8 machine. The only way to do it is to extend the taper further beyond the bearings. With the added bulk of the larger 40 taper holder, I expect the contact point of a standard endmill will be a couple of inches farther away from the spindle bearings. This makes a good recipe for chatter and deflection.