Pretty sure you can drive a car with blue plates for shit that isnt engine related, like seats. But when you get into 'performance engine upgrades' its a no-no.
Nice score. If only they were all still this cheap.
I'll have adjustable red Koni's and Celica ST Ultra Low springs to suit the rear available for sale in a few weeks.
7K block is wider so the 3k mounts may not work. The K50 box will work behind it but might not last long depending on how you drive. A T50 will work but you will need a K-T bellhousing which arent cheap or easy to find.
You can use corona disc brakes. There a few write ups about it. Best bet though is to save your pennies and buy a disc brake T series. Then you have the option of LSD's which arent available for the borg warners.