As above the swb would have most likely had 6.00 or 205's tyres originally, however I would fit 7.50's they are what I have on mine. Similarly originally they would have been tubed, but later defender tubless rims look the same and are the same size so it depends on what your preference is. Rangemaster were an original tyre that is more road based, an original all terrain would have been goodyear xtra grip, which you can now get copies off as a deestone extra traction. and a full off road tyre would have been the general super all grip, which from memory someone is remaking but I cannot remember who.
There are lots of others to choose from in a 750 size (I have michelin xzl's) and if you want a metric equivalent although not quite the same size 235/85/16 is the nearest and open up a huge market of different tyres. If you end up with tubed rims make sure they tyre is suitable for tube though.