I saw a GT the other day and looked at the parcel shelf, it already had a cutout for a speaker in the parcel shelf. but it also had the parcel shelf trim over the top of it, that wasn't cut out.Have a look in you boot on the left hand side. I will be interested to see if yours has it. The speaker shape is an oval type, smallish, and only one, remember you only need one for AM as it's Mono. IF and only if you can find the original speaker and it cover which was obviously the optional extra i would fit it in the parcel shelf. IF NOT, under the seat is good.
Re the idle, yes you could drop the idle down a little more to 1000rpm but as Josh said it is good for the oil supply to have those revs up a little, and hence why the idle was set at 1200 by your mechanic. It is also going to help with airflow in traffic.
Stick with points, no need to go with any electronic dissy's. as long as your have your car serviced regulalry, as you should, your points will be fine. after all points have been used in minis for over 40 years and they still are. I personally wouldn't go a pulsar dissy. Original car, orignal dissy i say.
The rust proofing you see on new cars would be ok to put on yours too, but it is unfortunatly messy and never applied nicely. if you go to a sale yard and have a look under the bonnet and boot areas you will see great big smears of gunk, thats the rust proof..... i would do it, unless i can do it my self PRIOR to paint in a resto. Yours may well have had that done during it's resto. Might be worth asking the question. If not, then i suggest you keep the car clean and chamoi it off after rain. if it happens to leak, lift the carpets/underlay and dry them. Keep and eye on the rust prone areas for signs of rust and attend to it as soon as anything is discovered. These areas include, but not limited to foot wells, wheel wells and seems.
A quick once over when you fill up the car never hurts. Call it a mini inspection. Check for rust, check your oil and tyre pressures, check your tyres for wear. check your washer bottle is full. check for oil leaks. all this takes less than 5 minutes and will save a lifetime of i shoulda coulda woulda. This is advice everyone should follow.
In regards to your floppy qtr window, buy a big bull clip from the new agents. grab a small peice of cloth(so as not to scratch the window or paint) and attach it to the qtr window at the bottom at the potistion you want the window open. it'll help hold in in place. to tighten them up, you have to get inside the door if i remember correctly, so if your not confident then let your mechanic deal with it at your next service. But the bull clip will get you by.
I met you breifly at the MCCNSW meeting the other week and i am looking forward to seeing your car too. Your partner's GT was very nice, but i didn't get a good look at it either.

next time.
Cheers
Aaron