Ask Holden or Richfield for the right one, my T-piece (looks the same as the one in the mini kit but has the correct (
part number TP7) thread).
My capillary came from that same eBayist as sold James' fitting kit.
Here is that capillary, the fittings between the gauge and the T-piece are standard, hence "mini", the TP7 thread is on the male part of the T-piece.
So you need a stock capillary, a T-piece with a female and a male thread to match the oil transfer housing,
(part number) TP7 if it's a TR7 one, imperial if it's the original, and one NPT thread to match the engine bay end of that capillary, Richfield's comparison chart
HERE. Easiest way is to ring and ask them, they'll send the right stuff, don't forget the extension piece for the temp. bulb if you do the logical thing and fit a dual gauge, so avoiding losing others or cutting the dash, that's in the picture at the top line of those shewn on that link before.