My Toploader has the 14” tailhousing.
The measured dimension from the gearbox/scattershield interface to the first mounting hole for the Hurst shifter is 15-3/4” or 400mm (give-or-take)
Here’s my Toploader after being fettled by David Kee’s agent in Adelaide (S Aus)….
So....you're building an ERA FIA ? Tell us more
Cheers,
Glen