Long headshell required

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.

@koala can you achieve any alignment type with standard head shell, even if cart must be twisted a bit from parallel with head shell? Could be that would work ok despite not being ideal. 

Thanks again for all of your input and suggestions. 

To explain the scenario - EPA100 tonearm has a large/wide mounting base and it's going onto a plinth where the arm board position doesn't allow it to get close enough to the centre (unless I cut into the plinth).  It is close to the right point and sounds good but is not optimal.  My measurements show that an extra 10mm length on the headshell will get me very much in the sweet spot.  I have moved the cartridge in the headshell as far along as it can go and it improved the positioning.  I'm using an alignment disc template to determine where it needs to go.

I was thinking of fabricating something and what @imhififan has suggested makes it clear what could work.  Obviously need to manage the weight and balance

It's certainly not optimal but I want to do something for the interim.  I need a different plinth but that's not in the budget right now.

I will give this a go :)  thanks