Hi Jon,
You're correct in thinking that the "ET" values is the offset; it is the distance from the centre line of the wheel to the face that meets the hub - so in the case of the original BBS roundies the hub face is 35mm from the centre line of the wheel - meaning the hub face sits closer to the outside edge of the rim than the inside. The effect of putting on ET29 rims is that their centre line will sit 7mm nearer the outside of the vehicle. Do you know how wide the original BBS rims are? I'd guess 6-6.5" from the tyre width..
Assuming the originals are 6", you will effectively have the outside of the rim sitting 0.5" +7mm closer to the wheel arch (about 20mm) and 0.5"-7mm closer to the inner arch (about 5mm). I'd have though the 5mm extra would clear the struts on the inside of the wheel, but not sure aout the extra 20mm on the outside - best get the tape measure out..
mikereynolds on this forum has a tidy pre-90 valver running ronal turbos - don't know what spec the wheels are, though.? ?
Hope this helps?

Mike