I remember those days.... Firing up Munich's finest straight six M3 lump in the chassi for the first time. Sounds great! Ours has been on the road for a year now. We got a custom rad made, which wasn't tooooo expensive, as I was worried the rad you have may not fit, and didn't want to be left with an unsellable chopped about rad. Been through enough parts that I thought would fit and didn't quite! We used Marlin's exhaust, but in think they've stopped supplying them now. But we did have to get the manifolds cut down, as they were so low a dead worm on a flat road could have hit them! Looks like you're making great progress. One piece of advice (if you don't mind me giving it), I would give is that you may well need to louvre the bonnet. We got vapour lock on the 1.5 hour drive back from the VOSA centre using the standard 'solid' bonnet. There's just no space in the engine bay left to dissipate any heat, and not much draw when stuck in traffic, or not giving it the beans. We did it after the test to avoid sharp edges fail. You might be ok, and when we spoke to Marlin about it, Mark had had no problems with his M3 Evo Sportster apparently when that was built.
The speed and responsiveness of the M3 is quite breathtaking, and the noise, the noise..... Sounds like nothing else, anyone who hears mine, says it's the best exhaust not they've ever heard!
Gonna get ours painted this winter, it's still in hearing aid beige at the mo, but some people say they like it in that colour!
Look forward to updates, takes me back to those days of knuckle skinning and head scratching.
Great job, keep it up.
Feel free to pop over and have a look at my M3 E36 Sportster if you want.
Rob, Lechlade, Gloucestershire.