Does anyone know if the rear style thermostat housing on both the Zetec & Duratec engines would have enough clerance if one fitted a
Grp4 style large transmission tunnel?

Thanks
Craig

Rear water outlet on a Duratec engine is not a thermostat housing.
(Thermostat is in the water INLET on the passenger side above the alternator)
I can do you a very slim outlet which will clear the firewall regardless of which tunnel you have.

I have one in a MK1 Escort with manual tunnel.

Phone me on 0408 804 795 for details.

Unsure of the Grp4 tunnel dimensions but the thermostat housing would sit just below the bulkhead lip so I doubt it will make any difference.

I have the Raceline water rail on my Zetec and this is where it sits (sorry for the poor pic). I have the engine as low as practically possible and the top of the water rail is resting against the timber in this pic.