1978 318
A set of 340/360 heads will increase performance with the increased flow, but the larger combustion chambers will lower your compression to about 7.6:1....
I had a set of 67 273 heads done up with 360 valves... 1.88" intakes, 1.60" exhaust valves with porting... The smaller combustion chambers on those heads will bump up the compression about .4 points.... Then a .040" head gasket will get the engine to about 8.6:1 compression... With the porting and larger valves, the 66 273 heads will flow as much as a stock 340/360 head or more...