Well 300 HP to the wheels is around 360-380-ish HP at the crank, so to make it easy, that says the 360 or 5.9 route. The 318 would be a piece of cake for the low 300 HP range and certainly can go more, but the 360 is gonna be easier. The crank is good in either. Your early 318 will have the lighter connecting rods in it; fine for 300 HP IMHO, but maybe not for pushing 400 HP, but those are easy to replace with some decent aftermarket rods.
Just a few random thoughts to start.
Compression ratio is the lack in both the 318LA and the 360LA for cruising around and wanting to spin the tires at will, but that is fixed by piston selection.
Now to heads. Do you want cast iron ones for the look or for any other reason, or are aluminum aftermarket ones OK?
The Magnum or late 360 LA does get to the roller cam, which is pricier but gets away from the flat tappet lifter issues with needing ZDDP in the oil.
Cam is going to be pretty easy; not too big to keep low RPM torque for driving around. Fuel mileage is a factor on that; is fuel mileage a consideration?
BTW, 3.23 gears if you never exceed 60 mph is a bit low IMHO.
