any benefit to swapping heads?

OK
High compression or low compression 340
If High and you have marginal gears you can do it but you can also get a flat spot or bog which is no fun till the rev's come up then you get the 20 hp at the top end
Low compression 340 do not even think about it - velocity in both cases you have to get the velocity up with the bigger valves which means flow at a higher rpm
x2 on the amount of gain with stock exhaust- which is the bottleneck in SBM stock heads