Can anyone recommend the bits I'd need to buy to add oil pressure and
oil temp gauges to Mellow Yellow?

Another option for remotely mounting a sending unit.
http://www.42draftdesigns.com/product/vdo/42004.htm
This option also allows you to put two senders in one port.  I kept the
original oil pressure sending unit and intalled the VDO unit using this
kit.  Boston Bob has had problems with VDO's dual oil pressure sender
giving false low oil pressure units so I chose to keept the stock sender
for the idiot light in the instrument cluster.

jake
82 ac westy