will 360 j heads fit on my 273

1.88/1.60 valves wont hit the cylinder on a 273, but 2.02's will. Valve-to-piston clearance is a function of lift, so if the valves didn't hit before you should be OK.

If they are "J" heads they are most certainly not closed chamber.

What is the casting number?

With that cam you'll have no low end torque no matter what, so you may as well focus on upper RPM breathing. For this the bigger ports will help (so long as the rest of you intake and exhaust are adequate).

What pistons are you running? What is your compression?