1972 318 la

While I like the bigger is better idea, I also am with the build what ya got. Spending money on a 360 to start could have been a set of slugs and maybe a little more.

The Magnum engine will need a Magnum vertical bolt intake. They can not use an LA intake. They can not use LA heads due to the lack of oil holes through the block to feed the valve gear through the head. On a Magnum engine, oiling is done through the pushrods.

If you have a Magnum engine, just swap the cam, intake, distributor to an earlier on with vacuum advance and add a carb and headers. Stay with the roller cam and reuse the OE roller lifters and spider. (AKA, lifter hold down.)

IF you want to do the 318 for a inexpensive cost, add cam, intake, 4bbl. And headers.
The expensive part after the bolt on and in's are the heads. Leave them stock for 300 HP or port them well with 360 valves. The 318 will use more cam to get to 300HP than a 360 would and may push the power band area higher than you like.

Adding a 360 head lowers the comp ratio to much.

( Use a multi spark system to light the fire on ether engine.)

What is the gear ratio and tire size? Is a converter upgrade in the wallet?
What is the target of the build?