Hello Simon, Welcome.
Some first thoughts re an OT1600 berlina replica:
Many of the issues you raise are similar to solutions adopted for 124 pushrod engines with 850 gearboxes in Fiat 850 chassis.
A range of photos in this forum under Fiat Abarth 1300 S Scorpione, and Fiat Abarth 1300/124 are very helpful. These have the engine sling, and Abarth or PBS bellhousing mating 124 motors to 850 gearboxes, front radiators etc.
For 124 twincam and 600 gearbox: if you look on this forum under car restorations "Periscope 1300OT Koos Visser" there appears to be a Fiat twincam, mated via a 131(?) bellhousing and alloy adapter plate,to a 600 gearbox (4 and 5 speed versions available plus alternate final drives), thus using clockwise rotation of motor/gearbox. Guy would know the details of the construction of the vehicle.
Ther are also Guy's photos on forum of comparison between VW beetle and Porsche 911 gearboxes for similar tasks.
For 124 twincam and 850 gearbox which requires counter-clockwise (CCW) motor rotation, this can be accomplished by reversing the rotation of the 124 motor, by reversing the rotation of the distributor (block mounted version) relative to the crankshaft, by an amended auxiliary shaft drivegear and matching distributor gear of opposite-hand/mirror image helical gears to the standard distributor drive gears. PBS used to make these many years ago using now superseded US Ford escort gears, now most only available likely by having one fabricated. I have photos of these and other detail if you wish to email me. The distributor drive gears on the camshaft and distributor from a Fiat 242E commercial benzina motor (type 132AZ2C) are the exact pattern for replicating, but can prove difficult to source.
This reverse rotation version uses twincam motor, Abarth or PBS 124/850 bellhousing and imput shaft, 1438 124 flywheel / clutch, 850 gearbox 4 speed (or Bacci 5 speed non syncro type), and usually a better final drive eg 3.9 to 1
If you can find a copy of Greg Schmidt's "Fiat and Abarth Tricks" publication, available on ebay from time to time, this has a helpful outline on the isssues.
Hope this helps.
Trevor