It's like sqbsprite says, different durometer bushings side to side is not a good idea and the Nylatron bushings are extremely hard. I have fitted the Nylatron bushings from Richard and they did require a bit of fiddling with to insure a good fit. They were on the large side to accommodate casting tolerances and had to be sanded to fit. Not a bad job, but a bit tedious all the same.
I also concur with his assessment that you need to check the diff mounts. There are two kinds of IRS running TR4A/250/5/6 out there. The ones that have had the diff mounts repaired and reinforced and those that need it done. Definitely have that checked out.
If you are thinking of buying some of the rear suspension bits from Richard, the thing that should be at the top of your list are the adjustable trailing arm brackets (your alignment guy will want to hug you for having them as they make rear camber adjustment much easier), just be prepared to replace some of the supplied hardware supplied with them. The frame to bracket mount bolts were too short when I did mine for using the toe adjustment shims and the pivot bolts for the trailing bushings had an extended threaded section that allowed threads in shear loading (as an engineer I hate having threads in shear loading). I would up using some AN hardware from Aircraft Spruce for the pivot bolts. If I'm recalling correctly they were AN7-41A which a 7/16"-20 bolt, nominal overall length of 4 7/32" with a 3 9/16" grip length, undrilled shank and head. Being the somewhat anal retentive type, I also applied some Dow Corning molybdenum disulfide solid film lubricant to the grip area of the bolts.