will 360 j heads fit on my 273

Yes some good points.. Stick with the 273 heads, put in bigger valves. Compression and burn flow in cylinder will be better, you can even go smaller guides and find a taller and smaller stemed valve like a Chevy uses, then get your installed height set were you needit, go short duration on the 273 to save the bottom end torque, and you can go up to low 500s on lift. This is similar to how the D Dart was built. That should get you close to 250-270 HP with 10.5 pistons and good ignition and exhaust.