Not only do you have to worry about outboard clearance, but also inboard clearance -- and the front and rears have different inboard clearances.

I haven't got a clue about the BMW wheels' offset, but that's the place to start. A Volvo 240's wheels (if OEM, including alloys) have a embossed number on the inner side, for examples either 5.5x14x20 or "ET20". The third figure of the first example, or the "ET" value, are both 20, which is the proper offset for 240 wheels. This is a figure, in mm, that tells you where the centerline of the tire, or the tire-carrying rim of the wheel, is in relation to the mounting plane of the wheel on the hub. So you have to start by determining the BMW wheel's offset, which I imagine is very different from a Volvo's (I'm guessing, but I think the offset is entirely in the other direction)!

The easiest way to picture this is to start with a Volvo wheel's dimensions: figure your problem is to draw a diagram of the wheel (like a mechanical drawing), placing the mounting plane 20mm from the centerline of the wheel, and then (knowing the width of the wheel, e.g., 5.5in on a Volvo) properly place the wheel's edge (at the tire mounting bead) in your diagram.

Then do the same with a diagram, drawn to the same scale, for the BMW's wheel, and overlay it over the Volvo's wheel's diagram.

At least this way, you'll get a picture of what problems you might face.

Also, remember that when you get your adapters made, they have to have a hub edge that's precise -- I forgot the right term, but it's something like "hubcentric" -- because the Volvo's wheels are really centered by this edge on the center of the wheel, not just by the positions of the lug nuts.

Good luck.