340 valves in 318 heads???

With some porting 'mainly removing the roof kink' the 318 small valve exh will flow with 340/360 stock ex ports.

The ported 318 intake will flow what a stock 360 1.88 valve head does but with the small valve.

stock flow is like 165int/120ex cfm peak, but with simple porting [318 heads are easy] you can have the flow of a 1.88/1.60 j head=200int/145ex cfm

I recommend you keep the stock valve sizing unless you need new valves anyways, in that case maybe think about the future holds for this engine or plans for a bigger engine.

maybe just valve job/hardend seat these heads and call it good.