Magnum heads on a 74' 318 LA

The 302 head is OK. It shines when you put work into it. It is pricey by compare. What needs to be done on a 302 to really see them work we'll is install the 1.88/1.60 - 360 valves and open up the port windows to match your intake.

The Magnum head conversion is a nice set up but also has it's price tag. While you can use them as is, the following mod is a good one IF you plan to run a cam with some decent lift. I forget at what lift the Magnum head can go to before issue, but if I remember correctly, it's just passed .500 lift.
So;

Have the machinist modify the head for the amount of big lift you plan on running if need be. Add matching springs to the cam at this time.

Above, it is noted that you "Make sure the head is drilled for "LA" intakes." This is not needed but a good idea for a wider verity of intakes that will cost less. I can only think of two heads out there right now that are dual drilled and they may not fit your intended purpose or budget.
The Magnum intakes are prices higher since there is less demand. Though there are basically 3 intakes for the Magnum head. They are as followed;

MoPar's dual plane - idle to 5500
Edelbrock's RPM - 1500-6500
MoPar's M1 single plane the book lists this intakes rpm incorrectly IMO since it is an upgraded copy of the Holley Strip dominator. It has a 3000 - 7000 RPM band and is a good race intake.
A note though, Hughesebgines has there FI. Version of Edelbrocks RPM Intake. While it is cast by Edelrock and made for Huges, it is an excellent intake that has the FI bosses cast in and ready to go.

But the intakes are the only pricey cave eat of this kind of build. So, IMHO, getting a head that is dual drilled for either intake is not needed.

As said above, use AMC lifters, remeasure your pushrod for the new length and order them hollow for oiling.

As far as cams go, what is the rest of your set up? Trans, gear ratio, tire size, car weight, the intended purpose of the car?

Also remember that any "LA" cam you get, divide the cams lift by 1.5, then multiply that by 1.6 Magnum rocker ratio for the new lift.