Has anyone had any experience with doing this on A series?
Are there any sorts of gains to be had?
Cheers
Dave

Its easier to do when the head is off the block. No sure where you get the kit of different thickness washers to do it.

No need to use special washers. A-series engines use plugs with crush washers, just tighten them enough to index the plug. Article "How To Index Spark Plugs"

Most say it is good for "1% to 2%" power increase over random installation, but I've never seen any dyno proof

I read once that an old school engine builder said the popularity of this practice came from the days when they were using obtrusive pop up piston designs in the search for more power and had to do it to clear the piston crowns sometimes. In terms of it's potential for gains for other engines, it's no doubt small but for the small amt of time it'd take, it's peanuts. It's fairly easy to do with the head installed too - just use a texta (I think Americans call em 'markers' and nfi what Canadians call em) and draw a line up the ceramic insulator in line with the plug electrode, then when you tighten it, you'll know where the gap is aimed at. Generally the 'go' is to aim the open/gap section toward the centre of the exhaust valve, which ought to be the warmest section of the compressed charge (if only by a small margin) and therefore the best place to get the quickest start/more complete burn going.

You might not get 5hp, but you've certainly got nothing to lose by doing it. I wouldn't swear to it but there used to be a place in north melbourne in arden street called 'champion parts' or something like that, and amongst various other bits and pieces they sold shim steel and various copper washers, you'd probably be able to find a size that would fit the purpose if you happen to be somewhere near melbourne, or you could try each spark plug in each cylinder, till you found one that snugged up at the right angle in each one.