the 3.73 diff will be too low for the e30 318i tranny. Stock those cars came with a 4.10 diff. With that setup the revs will be 15% lower than a stock 5 speed ti. You'd be a lot closer to stock ti specs if you left the stock auto diff in place. The whole reason for this is that the gears are a lot taller in the e30 transmissions and 5th gear is close to .8:1 ratio and our e36's are 1:1.
if you're rebuilding the motor anyway, why not just rebuild your ti motor?
I'm not sure about the e30 transmission in the 318i that you have access to, but I've had issues with 2 different output flange sizes. This usually isn't a problem with those cars since you can swap the driveshafts between the 2, but in your case you'll want the small output shaft flange to mate with a 5 speed ti driveshaft. Also, you may have to custom fit shifter brackets too. I'm trying to think of all the hang ups of putting an e36 drivetrain in an e30, your problems will be the opposite of those... you can get info on that here
http://www.akgmotorsport.com/usm50swap.html