One suggestion:-
a) a reverse (left hand) rotation Fiat Twincam using a PBS type reverse rotation auxiliary -oil/distributor- drive-shaft
b) an Abarth - or PBS type- 1300/124/850 bellhousing
c) a Bacci type 137 5 speed -dog engagement - 850 based transmission including optional lsd, or
d) a 5 speed standard 850 synchromesh transmission with overdrive (currently available Italy)
Advantages are:-
no adaptor plates required for engine / transmission connection;
standard 850 clutch and transmission setups including mountings/linkages/ driveshafts etc used; cv driveshaft options available;
several final drive ratios available on item (c) and (d);
Disadvantages are:-
item (a) alteration of twincam auxiliary driveshaft to replace oil pump/distributor gears, both on the auxiliary shaft and at the base of distributor in block (PBS kits no longer available) ;
item (a) is only an option for block mounted distributors (not cam box mounted) and requires an offset inlet manifold (available) to have sidedraft carburettors not foul the block mounted distributor;
item (c) no synchromesh, "dog" engagement;
item (d) robustness unknown