All the Uno shells have the same mounting points, whether 3 door or 5 door. It's the mounting brackets that are different depending on the engine fitted.
If the Pacer 1400 is the sohc engine as fitted to the Tipo then it's an easy swap as Fiat made a 1.4 Uno anyway. How easy depends on what engine your Uno orginally has fitted. If it's a FIRE then you'll need engine mounts, exhaust, gearbox (driveshafts maybe?), linkages and the engine wiring loom from a 1116cc/ 1299/ 1301cc or 1372cc Uno to fit the 1400 engine in properly.
If the Pacer engine is not related to the earlier sohc non-FIRE Fiat engines then I don't know what is needed to make it fit.