You dont have any profile info on your location, so the following info is if you are in Australia.

Since the a series is an alloy head, i would assume that it has hardened valve seats, so unleaded is fine.

You arnt going to get any noticable increase running premium over regular, but the motor *may* run better on the premium. My little bomber prefered premium but ran fine on regular.
Since your motor is modified, it *may* need higher octane fuel, but just try and find out.

Give each fuel a try and see what you think. In WA i am pretty sure that all the fuels come from the same plant in Kwinana anyways, so brands are really irrelevent here, and are much the same accross the country.

Some people have mentioned that they see a difference between brands and type, but there isnt much in it really.