So the Alpine is over, and it's back to work.
Starter and alternator are back from the sparky (minor salt water damage ! - must have got wet in storage or transit)
On to the gearbox.
Selector mechanism removed, and compared against new parts from celica (c60) box :

(c60 parts at top, trueno c160 selector mechanism at bottom)
The good news : the celica end cap (which carries the reverse lockout) fits the c160, so no work required there.
Placing the two shafts side by side, you can see that they are close :
(c60 shaft at top)

They have the same major and minor diameters, but the c60 shaft has a longer splined section.
Up close, you can see that the c60 shaft has the middle roll-pin hole about 1 mm closer to the other.

I think there is enough slack in the mechanism to allow for the hole spacing.
And finally, the c160 shaft has one of the splines machined off (hard to see), while the c60 has not :

I can have the splines ground back to match the c160, and have the key spline ground off, but I need to ensure the mr2 shift linkage will fit the end of my selector shaft.
Here is the end of the c60 shaft (apologies for the bad photo) : it has a flat machined in about 20 mm from the end.

I've posted a question (and some of these photos) in the gearbox/engine thread about mr2 selector shafts, as it may be of use to others (http://www.mr2australia.com/topic.asp?TOPIC_ID=18548)
Nothing is as easy as it sounds, but I'm getting there slowly.