I have Defender wheels and 7.50-16 tyres on my SIIA.
The disco I steel wheels can also be fitted in a Series. I've had a set mounted in my 88.
I believe that almost every steel wheels can be fitted to the series except the Range Rovers after 94, Disco II and Freelander.
Std SWB rims are 15", tyres are 6.00 x-plies. You can replace them with std LWB 16" rims with 7.50 tyres, or after-market steel wheels as advertised in the magazines (15 0r 16" rims with 6-7" widths) on 205/75R15 ar 6.00s, or 235/85R16 or 7.50s as appropriate. The larger radius tyres will affect your gearing and speedo accuracy (under-reading).
Fitting 7.50s or 235/85s on a SWB set up for 6.00s would overead by about 10% as an estimate, but if you measure the circumference of the larger tyre, and subtract the 6.00's circumference, then divide the difference by the 6.00's circumference and multiply by 100, that will give you the percentage of indication error for any tyre.