The engine will bolt straight up to the gearbox. You will need a different flywheel and clutch to enable the starter motor to engage and get drive from the crank to the first motion shaft.
Kennedy Engineering can help.
You will also need to check out the clutch release mechanism.
The 914/4 mechanism will work as factory with the priviso that the release arm is in a different place to the T1 'box so you'll need to re-route the clutch cable.
I have no personal experience in making a 915 clutch work in a T1.
Here's one solution on Russ Fellow's car
Both types of gearbox will need the T1 chassis & body modifying. The 914/4 gearbox needs the diff. flipping (or you'll get 5 reverse and 1 forward!) plus a 911 style frount nose cone & gear selector rod.
But apart from that it's easy
(I've managed it...
)