The stock 70 318 heads would be my 3rd or 4th choice of factory heads for this.
First choice would be Magnum heads as they flow pretty decent and have an efficient chamber to begin with. There is a little bit involved in the swap, but it's no big deal unless you already have an LA intake and stuff.
Next choice would be 360 heads. Face it at 375 HP you need the bigger ports and valves just for airflow. These heads have a bigger chamber so add the costs of having the deck/intake faces milled to increase compression, plus custom shorter pushrods. Until the Magnum heads came out, this WAS the typical setup for a high HP 318.
Third would be the late LA "302" casting 318 heads. These were the best 318 LA heads, and have a smaller intake port which will help low RPM throttle response a little. These heads also have a smaller chamber which, like the Magnums will help compression without a lot of machining. BUT the smaller ports will hurt you at higher RPM more than they help down low, especially since a 375 HP 318 won't exactly be a stump puller.
Fourth would be the heads you have now. 318 heads is 318 heads, yours only have the advantage of no air injection port to plug. And, you own a set which counts for something I suppose.
Reality check time... Add the $350 for porting to 16 new valves, 16 new guides, 8 hardened exhaust seats, springs, milling for compression and the associated labor and you'll have $1000 into the heads. Now price out a set of assembled RHS or Eddy heads which will support 375 HP out of the box.
A 375 HP 318 is certainly doable, but a 375 HP 360 is easier, cheaper and will support more down the road. Something to think about early on in the project, do you want a 318, or do you want 375 HP? Also, if you go the Magnum route, start with a complete Magnum engine as everything is at least 20 years newer. In fact 5.9 Magnum truck engine cores are pretty easy to come by. Why not start there, and copy the MP 380 horse crate engine setup?
