Are you aware that the 5HP30 is a 34kg heavier than the 5HP18?
B40 is 282HP/400Nm. The B44 had more and was released with 5HP24 in E38 and E39. @95kg, HP24 won't upset weight distribution so much. (E34 540ia has stiffer springs because of the HP30's weight), it takes 440Nm stock and the extra 20kg may just drop the front an acceptable amount instead of slamming it
HP24 hasn't been done before either AFAIK. Have you considered using one? 14kg less means you will accelerate and stop (a little) quicker too.
B19 should bolt up fine to any B40/B44, and you would get a major improvement: the superb selectronic shifter (may be simple to retrofit if you already have the trans connected). Also easier/cheaper to find. So long as using it does not require a CANBUS network on board, you should be fine using this one.
I can imagine a lot of people would be keen on this upgrade for the selectronic alone- it'd make driving an e34 auto better than driving an E39.