Hi Kenchi,
Wow, your question as to which arm and plinth is best for your 301 opened up an incredibly technical discourse on the physics of arm board construction!
I am somewhat new to high level TT set up and Garland 301 rebuild.
And I certainly do not approach this quest/avocation from an intense technical perspective. I research and use common sense. I have been searching and waiting for a “proper” rebuilt 301 rig for years.
Finally, I have what I believe is a great sounding set up. I can only share what I have been able to cobble together. Speaking of waiting, I still have not been able to listen to my 301 now waiting for the last critical piece-a Stealth Hyper phone cable which is still being built.
So, I can only relay my feelings and faith. For the arm, it is a Ortophon 309. Not the highest level but, I believe, period correct and really good. The plinth and TT rebuild is from Chris at Woodsong Audio. Believe he is one of a handful of rebuilders at the top of the game. And a very helpful and knowledgeable fellow. His plinths seem to be among the best. And to complete the picture, going with my taste for beautiful mid range a Myajima Shalbi. Hope this has been helpful. I hope that all my research, patience and common sense will result in rig that faithfully reproduces the drive and musicality that the 301 is famous for.