Combo opinions...

Main problem with 318 is cam & compression 340/360 heads just make compression worse, a closed chamber magnum type heads like EQ's would be a better over LA 340/360 open chamber. 273/318 come in closed and open chamber heads but even open chambers are smaller then 340/360. You have 302 heads port them put in 1.88-1.60 with performance valve job and some milling to raise compression it's all the heads you'll need.