Been researching this myself and from what I can surmise:
- 4k is the most often used, needs an adaptor plate which can be purchased pre fabricated for varying prices overseas. May or may not need a new gear idk.
- 4ac spins the opposite way but some sources seem to say that is due to the dizzy gear. Would need the same modifications as 4k dizzy, and is rather bulky.
- Most people seem to just get a older style 4ag dizzy and lock it at a certain ignition advance with an aftermarket ignition module, eg bosch.
I'd link you the pages I got this info from but cbf. Need someone who has actually done it and knows what theyre talking about to reply. There was a guy in brisbane city whom I bought parts off, who had a single weber downdraft and locked 4ag dizzy on his ae71, pretty cool.
- Also there are other aftermarket electronic options that offer ignition advance, and there is one for 300 aus on ebay from nz that offers ignition advance of the stock 4age dizzy.