Hi Reece - many of the single cabs (not all) are short wheel base, and all of the factory double cabs are long wheel base.
Also the cab mounts and deck mounts are different.
This said, moving the cab mounts should be no major problem, and deck mounts are a doddle... just buzz 'em where you want 'em.
All you'd need is a new rear driveshaft, new cab mounts and a bit of patience (read: several dozen beers, nice hot long weekends and a welder

) ... Well, you might need a bit more than that, but it is certainly do-able, and not as hard as you might think - the trick bit will be lifting the body high enough to roll the old chassis out, then the new one in...
Take photos, I'm sure we'd all love to see.
Oh, and do you have the front driveshaft and front axle in that running gear? I could use one of those...
regards
Steve