They are underpowered, cr1p handling rot boxes...but have a bit of charm and shiny chrome bumpered ones like your should be can make reasonable bucks when done.
Think you can even by a new shell for them, is it Heritage or someone that makes em?
I am guessing its a hard top by the GT, shame as roadsters are more sought after.
Like all these things you need to decide on your motives...
If you want to make some money on it, DO NOTHING TO IT, other than wash it, valet it, pump up the tyres, start her up, pre mot test, and really good ebay listing...classified of course.
If you dont want to make any money, but want an interesting project that will probably cost less per evening than going to the pub, and have a fun thing to show for it at the end then go for it. You should get a proportion of your money back on resale, unlike going to the pub, but I wouldnt go into this thinking that you are going to double your investment by restoring it, because I dont think it would happen. You would be lucky to get your investment back, and bugger all for your time, cuts and sweat.