If you're starting from scratch look for a 5.2 or 5.9 Magnum. Many of the good points were covered above but to recap:
Magnum heads have a much better combustion chamber meaning more power from the same amount of air-fuel.
Magnums have roller hydraulic cam so no worries about breaking in a new flat-tappet cam unless you regress. And you can get the stock cam reground to a hotter profile while keeping the stock roller lifters.
Magnum blocks/cranks/rods are equivalent to the LA motors in strength.
A stock Magnum will have the "beer keg" fuel injection intake but you probably will go to a carb if this is your first effort.
Magnum blocks still use a regular distributor, you can replace the stock non-advance one with the HP distributor of your choice.
Magnum stock exhaust manifolds are pretty good.
I believe Magnums have lighter pistons and thinner rings for less drag.
Magnums have a consistent compression ratio. Older Mopar engines usually had actual compression ratios between 0.5 and 1 point lower than specified. So this means your Magnum engine will have a compression ratio very close to advertised which I think is about 9:1, a good ratio for pump gas.
Transmissions and flywheels bolt up.
5.2 Magnums are neutral balanced like all other 318s. 5.9 Magnums have a smaller amount of external balance than 360s. Even though in real life external balance hasn't presented a problem for the millions of externally balanced American V8s out there, having less is easier on the crankshaft.

So, if you are:
1. not planning to buy aluminum aftermarket heads or,

2. paying good $ to have LA heads ported, with their crappy open combustion chambers,

you are better off starting out with a Magnum.

R.