It would almost certainly have originally had 6.00x16 crossply tyres on 5" wide rims, and the speedo will match this. 7.00 tyres were optional, but you need the lwb 5.5" rims to go larger (it may already have these), and 7.50 was optional with these wider rims.
I would not recommend crossply tyres - the handling will be markedly better with radials. Tread pattern should match the planned usage. 
I would suggest that you consider either 6.00 or 7.00 (6.50 was commonly used, but I don't think you can get them). While 7.50 is OK on your swb, you need to have a lwb speedo to match, and there is probably not enough advantage (if any for your use) to justify this. Metric sized tyres equivalent to these sizes in overall diameter can also be fitted, but watch the width relative to your rim width - the tyre supplier will advise.
Unless you are doing hard core offroad, there is no advantage and some disadvantage to fitting longer shackles. Apart from the cost of engineering the modification, the change in angles on the Universal joints will cause increased vibration and wear, and the change in castor angle will not help steering. 
Any of the tyres mentioned will fit without any changes to the vehicle (with appropriate rim width) except for adjustment of the steering stops to keep the tyres off the springs and chassis. And here is the major advantage of smaller tyres - you get a better turning circle. Smaller diameter tyres will also give a little snappier performance in traffic, but its not going to be good by modern standards no matter what you do, so I would not worry too much about that. Similarly, the larger diameter will give slightly less noisy cruising speed, at the expense of less performance on hills, but again, the difference will be pretty small, and neither the noise nor the hill climbing at speed are anything like what you are probably used to.
Hope that helps
John
				
			 
			
		 
			
				
			
			
				John
JDNSW
1986 110 County 3.9 diesel
1970 2a 109 2.25 petrol
			
			
		 
	
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