Looking at the picture you see cross sections of the 688 engines.
Both engines left side is facing towards the front side of the car and thus the engines are reversed
Though in the base both engine are the same there are differences as early mentioned.
Waterpumps are different, the Estafette has a larger one which propelles the radiator fan, where the GTL/F6 have a electric operated radiator fan.
Distribution chan covers are different, manifolds are different, the engine block casting is different as
@Andy McGhee wrote.
Estafette has pulley on crankshaft to drive the generator and waterpump so there's an oil seal in timing chain cover, GTL/F6 has pulley on the camshaft so an oil seal at camshaft end.
I would not use an Estafette 688 for my GTL/F6
The standard Estafette engine won't even fit.
