Honestly - put the 3.2 back in the 156 (or a 156 2.5V6 donor car if GTA damaged) and drive that. You're making a rod for your own back if you're trying to take a car that is CANBUS and turn it into a non-CANBUS car.
Because in a CANBUS car nearly everything, down to window switches even, operate on the CANBUS network, each item you touch, replace, re-wire or mess with is going to have a knock on implication for probably half a dozen or more other items. It'll drive you crazy. People simply don't do it.
As you've seen, folks are shocked that you've even managed to get it running because the CANBUS network usually has a horrific tantrum and stops anything from happening.
As described above, you're only likely available option is to take a 147 GTA 3.2 instrument cluster, ECU and body computer.
Try looking into the Alfa GT 3.2 - did you have those in your country? The 147 and GT are very similar bits of kit, so it may be possible to use GT parts of 147s not available, you'd have to look into it more.
1999 156 1.8TS Sport Pack 3
2001 166 3.0 V6 24V Super
2003 156 GTA 3.2 V6 24V
2005 156 1.9 JTDM-Jet Sport
2008 159 1.9 JTDM Lusso
1997 GTV 2.0TS Lusso
2001 147 1.6TS Lusso