Standard tyres on it would have been 6.00x16, optional 6.50 or 7.00x16 cross plies. 
This is the largest you can fit on the standard wheels, and you should note that those wheels do not have the safety ribs normally expected for tubeless tyres, although in fact tubeless tyres have been used on them without problems. 
The Series 1 had a relatively low powered engine by current standards, and increasing the overall diameter of the tyres will not help acceleration, although it will be a bit quieter at higher speeds - if you ever get there. Larger diameter tyres will generally help offroad capability, but with the standard tyres it will be right up with anything you can buy today, and ahead of a lot because of the light weight.
In my view, for this vehicle, wider tyres are not an advantage in any respect, particularly since any significant increase in width means new wheels. You can fit the wheels off the lwb Landrovers from Series 1 until Defender, and these will allow wider tyres, but bear in mind your state's limits on tyre size increases, and the fact that there is no power steering. Wider and larger tyres will also make more evident any shortcomings in the steering and suspension. I would probably fit 7.00x16 if I could get them at a reasonable price.
Radial tyres were never fitted to Series 1 Landrovers, but were available on Series 2a. They generally improve driveability, and probably most non-purists fit them. 
Type of tread - up to you, depends on what you are using it for. 
I can't help with what is available, partly because there are tyres that you can get from your local supplier, and then you can get tyres from specialist suppliers (usually very expensively!), so there is in fact a very wide range available.
John
				
			 
			
		 
			
				
			
			
				John
JDNSW
1986 110 County 3.9 diesel
1970 2a 109 2.25 petrol
			
			
		 
	
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