I used the stock E34 525i intake system, including the intake elbow and intake box - although I used a rotary tool to widen and radius the inside of the intake box to facilitate the increased air flow from the larger engine and I did also retain the S52's CCV system
There was some complication when it comes to retrofitting the S52 CCV system to the M50 manifold. The most proper way to do this is to use an M50 manifold adapter kit and weld a return bung onto the E34 dipstick tube for the CCV return/drain hose and maintain the S52 CCV system. That is what I eventually did, as pictured below before the dipstick was painted
Alternately, you can rig something up to work without the expensive CCV retrofit hose but this always looks a little janky (as below) and there are other compromises that you have to make to do it like this
Installing the M50 manifold on an S52 requires either using an M50 throttle body or an adapter plate for the S52 throttle body. I also chose to maintain the stock M50 throttle body cooler and it's associated thermostat mounted in the intake box. It is very easy to bypass this system, but my goal here was factory functionality and appearance, so I chose to maintain it. You can see the thermostat hanging out around the bottom, coming from the hoses to the TB
As far as exhaust goes, stock E34 headers will bolt to an S52 and the general consensus is they are an upgrade over stock S52 headers as the M50 primaries are a larger diameter
For the fuel system, I swapped the E34 fuel pump to one from an S52 powered car, while maintaining the stock E34 housing. No further modifications were necessary because M50 and S52 receive fuel in the same places so even fuel supply lines are the same
Note: the S52 era cars use a fuel regulator valve near the fuel filter, whereas E34 M50's used a fuel regulator valve mounted on the intake manifold. Because I swapped the M50 intake manifold onto the S52, the fuel regulator valve is not an issue, as it stays in the original location. As such, if you want to maintain the SS2 intake manifold you will need to install a fuel regulator valve near the filter, like the E36 S52 setup
One additional thing I did on this build was to open up the stock intake box a little bit to allow for more airflow capacity. On the bottom of the stock box is a "horn" which is pretty much just a restriction from the full diameter of the hole down to maybe 1.5-2" wide. I completely removed the "horn" and then rounded out the opening there
The top of the box features a trumpet and though it seemed slightly less restrictive I chose to open it up slightly, essentially cutting half of it off while smoothing things out as much as possible
These changes were completely optional but I did them to maintain the stock intake box while hopefully also allowing better airflow for the .7 liter larger engine