I have the B+G 40mm springs in my e30 also,
they will drop a bit with the heavier engine. mine was originally a 316i but the cam belt stripped on in 4 days before it was booked to get it replaced.
i put a 1.8l in it and the front dropped another 30mm extra with the added weight.
it gave it a real staunch stance being nose heavy.
pic is of mine and my dads car side by side. badd