Long headshell required

You are right, @solypsa. Good catch! But my point is the same - increasing overhang won't offset a misaligned P2S distance.

Thanks for all of your suggestions.  Since I'm not prepared to cut out the plinth, it's the best way to improve the P2S distance.  It wont be perfect but it will be close. 

Does anyone know of a long SME headshell?

Changing the overhang will not in any way affect P2S distance. I don't think the headshell you seek exists.

Something like this might works:

https://www.audioadvisor.com/prodinfo.asp?number=VPHW

You wrote, "...I need to use a longer headshell to fully optimize the alignment."  What alignment are you seeking to achieve?  Very often, if the tonearm headshell offset was chosen to adhere to one algorithm and then the user tries to align to a different algorithm, the cartridge cannot be pulled far enough forward in the headshell.  Is that what is happening?  One would need to know what tonearm you are using and what alignment you seek to achieve.

While others suggest you cannot compensate for inaccurate P2S by altering overhang, in fact I think you can, but the relationship is not simple arithmetic.  For example, if you have a tonearm with 250mm effective length, where effective length = P2S + stylus overhang, and P2S is 235mm, overhang = 15mm, and if you mount it so P2S is slightly incorrect, the new overhang is not simply the number of mm required to achieve a total 250mm effective length. And it depends upon what algorithm you are using.