To the best of my knowledge the boxes are the same externally apart from the rear diff casing which varies between the 156/GTV platform & the 166 platform due to the position of the rear engine mount.
Internally the 2.5 box has a lower final drive which many prefer in the replicas because it give the stronger acceleration at the expense of a taller cruising gear.
The release arm is the same on both boxes & the boxes are directly interchangeable. The variation being discussed here is the slave cylinder bracket which is definitely different between the 166 & the 156 boxes. The 156 boxes have the solid flywheel & therefore the flywheel & clutch assembly is quite slim so the release arm sits farther round on it pivot to reflect this & the slave then sits more parallel to the centre line of the car. The 166 runs with the dual mass flywheel which creates a much deeper assembly, the release arm then sits farther round on its pivot which means that the slave has to as well hence the fabricated steel bracket found on the 166.
The cast alloy bracket from the 156 does now clear the gear mechanism but Phil's issue will be rotating the release arm on the splined fork shaft to make it realign with the cast alloy bracket as he still has the dual mass flywheel installed.
See HERE for the required bracket.
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