If your BB is 68 mm wide (or even 69), that's a standard width, and so any spindle/crank combination that's made for a standard bike will work fine. Unless you're doing internal gear hub, or singlespeed/fixed gear, where chainline is more critical, you can get close enough if your spindle and crank put the middle of your chainwheels (1, 2 or 3) at approximately the middle of your cogset in the rear. If you have a specific crank, then you can look up (or your LBS can) what spindle length and taper-end-standard (ISO, JIS, Campagnolo, etc.) the crank should ideally work with. How a square taper crank actually fits on a given spindle is, unfortunately, somewhat of a black art -- manufacturing tolerances, whether the sockets in aluminum crank are worn a bit, how tight one torques (or has torqued) the crank bolts are all involved. But that's a general problem, not one that is particular to your Raleigh.

Before you do (or at least buy) anything, do check that the threading is indeed 24 TPI rather than 26. If you're lucky, the cups on the existing BB may be marked with the threading -- some models are "marked" by the presence (including the non-presence!) of a certain number of rings incised into them. Lacking that: if you don't have an English thread (1.370 x 24 TPI) cup to try out, borrow one. If you can thread it all the way in, then you're all set, except for the spending part.