I can confirm the Michell Technoweight will work on your RP6....I have one on mine. The stock end stub is very tight, you'll need to wrap it in tape and use some pliers to break it loose. Just be careful, support the arm, etc. I switched to the Michell for the same reason you described. I'm running a 10x5 and with the spacer the weight hit the cover. The Rega tungsten weight should work as well, it's quite a bit smaller.
FWIW, I'm really happy with the Michell. Definitely tracks better and I had a noticeable increase in bass response.
I can't imagine there is any reason the Groovetracer won't also fit.
Dynavector 20x2H | Rega RP6 (GT Ref, Technoweight, M.L. Dual Pulley, white belts) | Jasmine LP2.0 SE | Line Magnetic 211IA | Reference 3A MM De Capo | REL T5