Which tonearm?

I have a VPI Hw-19 Mk III with an armboard drilled for an RB300. I want to upgrade my tonearm but VPI hasn't written me back about armboards for the HW-19 and I really would rather not get another one. So if that's the case what tonearms can I get that would fit the armboard drilled for an RB300? I did read here that the JMW-9 has the same geometry but he had a Scout. Being a newbie, not sure if that applies to my turntable or not?

Another quick question. If I were to upgrade to the mk IV which has either a TNT platter or Scout platter, would I have to change to a thicker armboard because they're thicker platters? Thanks for your patience!