Well, those pulley sizes sound pretty close, so maybe it will be right-on.
If you're successful with this, you could probably get a very small cottage industry going making replicas -- I bet there are at least a half-dozen 240 owners I could think of who would be excited to buy a plug-and-play diesel tach!
I have a couple of spare 700 series diesel tachometers; although their accuracy is nothing great, if you would like to get a calibration reference specific to the Volvo application at some point after you get this working in order to dial it in, I'd be happy to loan you one by mail -- contribution to R&D.