The GMC 270/302 head came in 4 different designs. If you use a flattop piston with that head your compression will be about 5:1. Do a search on Google and you'll find all the info you need. The best head to use is the #983 with a custom piston.
My 321" GMC has Hudson rods (1.115 inches longer with a custom JE piston). The CR is 11.5:1.
Ron
