318 cylinder head

I have always gained by replacing stock 318 heads with 340/360 heads, especially when milled .040. Always.
I've never had to cut the intake when I milled the heads that amount and never an issue. My experience, yours may differ
You can work with the 318 heads and get modest gains as well.
Yep, aftermarket heads work nice as well, though a set of (per say) edelbrock aluminum heads won't achieve potential sitting on a stock 318 long block with just a 4bbl carb. This would be my last choice if you plan on leaving the 318 otherwise stock.