Have you tried original seats ?
I have the original seat backs in mine (from '56) but the bases had have re-made many years ago.
Drove from Melbourne to Cooma in 2008 without issue, not exactly uncomfortable but also not exactly comfortable either. We stopped & changed driver whenever the fuel tank needed filling so got a brief leg-stretch. 
Changing the seats impacts on the resale value and unless it's going to be a daily driver why would you change them ?
Worth trying some in someone's Series I, you could change foam density or use two different densities so it looks original but is more comfortable.
Colin
				
			 
			
		 
			
				
			
			
				'56 Series 1 with homemade welder
'65 Series IIa Dormobile
'70 SIIa GS
'76 SIII 88" (Isuzu C240)
'81 SIII FFR
'95 Defender Tanami
Motorcycles :-
Vincent Rapide, Panther M100, Norton BIG4, Electra & Navigator, Matchless G80C, Suzuki SV650
			
			
		 
	
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