Personally I use twin air filters, my uni filter only got used once before a hole appeared at the bottom of it caused by a bit of fuel sitting there dissolving the foam
The 74 onward bikes that only run a single plug use the rear hole, I have 1 for a 74 & it is very obvious that the front hole could be drilled but wasn't at the factory.
The way the transfer ports work may have something to do with the rear plug being preferable
I'm running a rd400 coil on my hybrid ignition firing both plugs, starts first kick every time but for the initial start of the meeting which seems to be more fuel related than anything