Importance of arm length?

Here's how I understand it.
Every pivoting tonearm travels in an arc as it moves from the perimeter toward the center of the record. As it arcs, it loses speed on the first half of the arc and gains on the backside. Tonearm designers create a mathematically calculated offset in the form of a bend or angle so as to minimize the effects of the arcing. In order to do this they need a constant effective length so they designate one. It is measured from the pivot point to the stylus tip and on some models can be adjusted by moving the cartridge to or fro in the headshell. This is the overhang adjustment.
Unless you have a replaceable armboard on that turntable, I would not drill a second hole in the plinth as that would be unsightly. Why not investigate available tonearm options to learn which ones can be used in your existing hole. The MMT is decent but you may be able to improve upon the performance it offers without spending a lot of money or defacing your prized new turntable.

VPI no longer supports the HW 19 tables but you could contact them about buying any plinth they have left over. The plinth from those tables is a laminate of several materials if I remember correctly.
You sound like you are handy and, if that's the case, you can use the parts from the VPI to make your own table out of any material you feel like messing with including Corian or granite. On the other hand you could just extend the hole in your current plinth so the it extends back about a half inch. There is about 7/16 of an inch difference in effective length that you lack. If you were to drill the arm hole so that it was a long oval, you would have enough length and the base collar of the arm might even conceal the extra opening. I have seen designs that leave room to slide the arm mount for adjustment. Should be fine.