A1 must be with A2; D1 must be with D2
B2 and C2 could fit in B1 or C1(some sculpts could only fit either B2 or C2)
A3=B3=C3=D3 (hole size)
A4=B4=C4=D4 (ball joint size)
ps. "=" means approximate, not exactly the same.
So, prepare some B and C necks for different head sculpts.
A1< A2(hole size)
B2=B3 > B1
C1=C3 > C2
D1=D3 > D2 (hole size)
So, what can't we do is A1+B2 or A1 + B3
C2 must be with D2 and C1/C3 could be with D1/D3
If you want A2+B1, then you must cut B1 to the length of B2 and use a tape with B1 to be as thick as B2.
You got it？