In theory as things stand you can only manage 35 turns of 2 in hand 1.6mm and that is raising the cut in too much and will knock things in the 12 mph region. For you to be able to use the 1.6 wire you need to get some more turns in without loosing too badly. it will no doubt be a struggle to get the equivalent of the 1.4mm wire.

If you increase coil thickness a lot you drop the flux density and it becomes a chicken and egg situation. Also reducing the coil hole size will partially offset the increase in turns.

On balance I would keep the coil hole magnet size on the outside but squeeze it at the centre where you need to gain the space. if you can't get the number of turns with triangular coils you will be forced to increase thickness, if you are very careful and get things true you may be able to run the thicker coils in the same air gap. Increasing air gap 1/10" will not drastically drop the flux.

It's a gamble but worth trying if you have plenty of the 1.6 wire.