Pulling the heads is a good idea. as well as using oil to try to loosen the piston rings. personally I would drop the pan and make sure it doesnt have a spun or siezed main or connecting rod bearing as this will also lock up a motor ( be a waste of time to mess with the cylinders if this has happened. having owned this same car I can tell you a few things about them –
Don’t expect to much out of the 403 , low hp motor although it did make decent torque for what it is. the ws6 suspension is really nice. handles well and stops good with the 4 wheel disc brakes. throw the carb thats on it in the trash. an edelbrock performer carb ( 625 cfm ) will work great. if you want the car to fly p/u a good 455 ( the earlier years the better ) build that up and drop it in ( the earlier t/a’s came with a 455 ) for a trans go with a 700r4 ( you will get decent gas milage and the low first in the 700r4 will let it launch )