If you're using T51 toolholders from RDG, (the ones I bought from them were made by HBM,) they may not fit an existing T51 toolpost! I had a Boxford 280 with an original Boxford T51-sized toolpost on it, but acquiring toolholders to fit it was not really an economic option. I bought a complete toolpost-and-holders set from RDG and I've been perfectly happy with the set-up. I usually bought a couple more holders each year at Doncaster. (Exhibitions… sigh. Remember them?)
The original Boxford toolpost is made to tight manufacturing tolerances, such that the eccentric cam only has a small (approx. 0,5mm) eccentricity, enabling it to get a good grip on the toolholders with a relatively light tightening torque on the clamp handle.
The RDG set-up, although apparently identical in size, has a greater eccentricity (Approx 1mm) so manufacturing tolerances (= cheaper to make) are relaxed. Hence the RDG clamp handle has to be given a bigger heave to tighten it than the Boxford one. So what?
I later modified my original Boxford toolholder cams by grinding bits off them to get the 1mm eccentricity, and was able to get the RDG holders to fit it OK.
(Before modifying the cam, I had originally modified the clamp head – the Tee-section bit that gets hold of the toolholder – on the Boxford toolpost by thinning it down so it gripped an RDG toolholder OK. Great! Unfortunately… the next toolholder I tried wouldn't clamp up at all – I'd taken too much off the thickness. Grinding the cam and using a putting-on tool on the clamp head sorted it, and I sometimes use the Boxford toolpost plus RDG holders on the Myford on a special mounting block directly on the cross-slide. I wrote about it in ME a while ago.)
Hence if you're wanting to buy inexpensive toolholders, you may need to modify your toolpost so they fit. Or buy the toolpost as well, like I did.