Combo opinions...

On the heads check out 1wild&crazyguys sticky on porting 318 heads with the factory port size & valve size and stock valve job they flowed 175 cfm which is good for 350HP ootb and after porting he got 210 cfm with stock valves and valve job which is good for 420HP since your only look for 300Hp I wouldn't worry about the heads other than 3 angle valve job and a very minor clean up of the ports but if must go bigger valves go 1.88-1.60. Plus there's an Engine Masters article comp cam XE262H with 4 barrel and headers on a stock long block made 282@5000 rpm probably would of made 300hp with higher compression.