Sounds like you're sitting on a genuine dogleg gearbox - which is pure gold IMHO.
Few things:
- get yourselves prepared to have someone doing some aluminum tig welding.
You won't find a shifting arm that'll fit - I found myself a M40 shifting arm (which is quite long)
and had it shortened to this:

- get a long diff.
5Th gear on a dogleg box is prise-direct (this is one perfect way to verify if you have a dogleg box anyway); which means your input shaft will be running at the same speed as your output shaft in 5th gear. On overdrive gearboxes 4th gear is prise direct.
I chose a 2.93 diff; which I still think is the perfect combo. 2.93 diffs were used on all e30 ETAs.
The long 1st gear will kick you in the butt - without having you just burning your tires; 2nd will have you just passing the 60 mph mark; and 5th will bring you a 145ish top speed.
- whatever you do; make sure you attach either the M3 bow or the shifting plate to the gearbox before you get it in the car.
Ask me how I know...