Basically the whole setup will be the issue, 1200's are so light it is hard to load the turbo up to make decent boost,Even Factory turbos really start making decent boost from third gear onwards because the lower gears preclude making good boost pressure.

Taller final drive, Throw a couple of passengers in the vehicle that should make a noticeable difference to how it feels.

Having built a 1200 Ute with a Z18 Turbo it needed a change from 4.1 to 3.7 diff ratio plus changing the input gears in the gearbox and then really did not make good boost till it had a couple of cement bags in the tray of the ute (25KG each)

That is why Nissan with the March Super Turbo used both a Supercharger and a Turbo to make it work.

So with a 1200 you need a really small frame turbo taller gearing and good tuning to get a decent result, Not something you can just knock together and bolt on and expect results, Sorry but them's the facts