you would be best getting a maddat fitting kit, because you need to modify your engine mount on block and cross member also with other engine rubbers. and it also comes with a bigger sway bar that helps you out alot in the handling area.
i tried to fab and s13 crossmember but where it bends down to the part were the rubber mount bolt onto is right where you need to bolt it to the chassis.
you will also need to make a hydraulic clutch set up
with it being a manual you can get away with massaging the opening but because the s13 box is longer you need to open it up and extend it a little bit at the end and your shifter will be roughly 20cm back from standard shifter position.
you could use a MR30 gear box out of a skyline or a 71c out of older skylines bluebird etc
best of using the 71c because its alot cheaper and not that much different.
ive got a maddat sr20 fitting kit forsale for a couple of hgs cheaper and you dont have to wait on sending yours to maddat and getting one back.
that whole process can take two weeks.

but is a good little project