Cool, so I can use the dimensions of the Sparco Pro2000 as a theoretical maximum
Dimensions as follows
A: Bolsters Y Dia = 473mm
B: Shoulder Width = 576mm
C: Shoulder Height = 595mm
D: Cushion Width = 490mm
E: Cushion Depth = 520mm
G: Back Rest Projection = 115mm
B: Shoulder Width = 576mm

Any chance of a pic of it fitted? I only measured 460mm clear between the tunnel and inner door lip . So I'm assuming that you have the seat sitting over the tunnel a bit .