Just looked at my old invoice when I did mine in 2010 😳
I used a 64mm cutting bit and it was a tight fit. I part pushed it in then screwed it up the rest of the way. I also used a flange, as at the time I thought it would leak (and I'd constantly worry) plus water /heat resistant sealant on the thread.
I've got a 500ltr upright tank and I put the 11" immersion into it.
Also, you'll find that one you've nipped up the flange, if you use it, it grips nicely and you can tighten it up without the use of stilsons on the inside of tank.