Mark when an engines been overheated the timing belts usually least of your worries. Usually piston pick up fecked head gasket etc occurs. As Willie points out the screech is unlikely to have been the timing belt, probably due to the steam round the engine its been an auxiliary belt got wet and started to slip.
you say that after topping up the system the engine ran ok, i'd hazzard a guess an say youve been a lucky sod and got away with it.
Ive never had the pleasure of working on a Z32 engine ( yet!!!!!, still looking for my first real car :) ). but ive worked on plenty of other engines (usually a bit bigger than a Z though) and work to the principle that if it aint broke dont try and fix it, do what your doing and monitor the situation, if your loseing power, losing coolant, the oils turning grey and sludgy or if the cooling systems pressurising then yes look deeper. but if its not leave it be, sometimes when something is stripped down it just never goes back to how it should be.
Back to the timing belt I reckon that a change would really only be for peace of mind. that said ive been there myself, once you've got something in your head it knaws away until you end up driving the car at no more than 2000rpm. If your in this frame of mind probably then best to have someone look at the belt condition then you can get back to driving it as it should be driven.
Hey but you shouldnt listen to me many many moons ago i burst the belt on my pride and joy a 2.8 supra through pure lazyness ( i was at the time working as a mechanic and had all the facilities at hand to do a quick check or change) Luckily i got away with just fitting a new belt. I was too lazy or was that frightened to look for any damage to the engine. the car started and ran perfectly, I never looked back.
And Willie, have been lookin out for the red Z round town but havent seen it yet