Allright, time for an update now. Didn't do much except driving, made about 4000 km without any problems - besides a frozen battery two weeks ago. Car stood outside at -15°
Last week I spend some hours removing bitumen-coating in- and outside. Wish I hadn't..
Floor looked good... from the outside. There must have been an aquarium inside some time ago, the rocker panel rusted open below both doors and the floor panels are thin like paper.
Also discovered lots of rust at the longitudual member just underneath the leafspring socket in the back and tons of fibreglass on the front axle mounts. Feel quite uncomfortable thinking back of the last 4000 km..
After the new years brouhaha the car will be dissected. Rats, didn't want to start a restoration until the BMW is finished.

speaking of...: I found some mates here talking about a mazda mx5 rear axle conversion, is there actually anyone who made it? wasted mx5s are quite cheap over here and if i really have to build the floor and such from scratch, why not going for some improvement?