The biggest drama is the wiring.
With my conversion, because the R33 has the battery in the boot (and the R32 has it in the engine bay) the two looms had to be wired into each other - not an easy job.
Everything else in the conversion went really well, the engine dropped straight in and bolted up to my R32 gearbox.
I've got a R34 GTt side mount intercooler fitted, which was a struggle to fit in the standard location, but (because you are changing turbo's) I imagine you will be fitting a front mount anyway.