Nice options there, though what condition is the body in? Especially rust wise?
If you take a look at
http://www.oldride.com/library/1968_pontiac_gto.html you will find some good info there.
If you check on the front of the block there is a machined pad near the water pump. There you will find a stamped code begining with two letters such as YS then the last four or possibly five digits of the VIN. The first two letters will give you the info on the engine (check info on link above), and the letters will confirm if it is the original engine.
Hope that helps, i hope it works for you, it looks good from here.
Dave