nice man, i have the same setup from my old e34. I just purchased the exhaust pieces!!!
Magnaflow 98004 dual in dual out 2" catalytic converter
Stromung e30 m3 exhaust from understeer.com!!!! I can't wait to see how the 3.5 sounds on that system!
I'm planning on running the stock pipes to the magnaflow, then slipping 2.5" over the outlet and running dual 2.5 to the stromung. should be deep and loud!!!! with enough backpressure too i think
also buying the e30 shift carrier and reusing my e34 ssk cause aparently it will work. also found out the stock e30 driveshaft fits without modifications!!! it's exactly the right length i just have to make sure the carrier bearing is in the right place.
figured out the cooling last night as well. it was a little odd cause the e34 had an aux waterpump (electric) and 3 hoses to the interior heater area. the e30 only has 2. i had to make sure i had them right and then cut and clamp them on. hope it works.
also purchased a 14" permacool fan (high performance) 6 blade metal beast capable of pushing 2950 cfm through my radiator. that with some waterwetter and i think i'll be a ok.