When I was doing my 302 heads I discovered that there wasn't a lot of room under the spring retainer. In other words, with an installed height of about 1.600". the springs would be close to co8il bind with a cam with lifts over 0.500. But not to worry because by that time the retainers would be pushing against the valve stem seals. I'd make sure there was enough clearance and the spring was right. The best one as far as fitting in the space seems to be the Hughes 1110. It looked like it would allow the most lift of the single springs.

Steve Dulcich is the man for writing articles on the 318 and he has I think three different 400hp 318 stories out there. He's used 360. Magnum and 302 heads. The 302 heads required a lot of work and reportedly cracked soon after. Look at Steve's article "Junkyard Jewel" for more information. There is a lot of material in the top of the port around the valve guide, and the guide itself. The pushrod pinch is also a choke point and it is pretty thin so be careful.

Before you do anything, though, get a price so you can do some comparisons. ProComp has a cylinder head in the affordable range, Edelbrock and Promaxx have decent heads, higher money. I don't know what the Sidewinder heads will run, between the high and low, I guess.