well for me , regarding the leads first , the aftermarket buget ones seem ****e, I had arcing on my 3.9 with bosch,mirrortech and lucas so it has to be genuine(which it now is) or better.
For my 72 3.5 I could buy another set of genuine for between £30 and £60 depeding on where from but they are only designed to last 2 years and carry no warrenty worth having and people have had some dodgy sets through form new as british oem parts are well know for being ****e, or I could get some magnecore for £70 direct and they come with a 10 year warrenty
I think it will be the magnecore this time
Oh and rgearding air filters I have just bought some coopers ones for the 72 but looking at the size of each carb and the size of the single air inlet into the airbox that must restrict performance as does it sucking hot air in form the engine comparment (I know K&N would do that too)
I am thinking of rigging up some cold air induction to match my full stailess tubular headers and exhaust when that goes on!