It's your car. You're the one doing the work, putting the money into, and ultimately only have to satisfy yourself.
What do you want out of it? There's some pro's and con's to the B engine you've got over the 318. It's heavy and will provide a certain amount of push in cornering. Nothing really negative over the LA, but something to take into account if you're an aggressive street driver. There's also no replacement for displacement and if you've already got the 400 built then the engine mounts are fairly inexpensive and headers... done.
On the other hand, there's a lot to be said about the LA engine, too. Wouldn't be hard to take it to 390" and make one helluva a sleeper. When you blow doors off and someone wants to know what's in it you get to tell it's a 318 with an evil grin. The LA will, of course, keep the weight off the nose, making corning a lot less of a chore over the B engine.
Whichever way you go, take a look at the rear. If it's a 7.25" in there, you're going to want to upgrade it to at least an 8.25" and build that for any serious horsepower. 8.75" B-body rears are damn near a bolt in and not too terribly expensive and ready to handle the horsepower. And while I'm not a fan of the Ford 8.8" they're cheap, widely available, and disk brake ready.
I pretty much advocate the need for suspension upgrades on all F/M/J bodies. Eliminate the rubber where you can to help put that horsepower to the ground. Swapping rears gives you a good excuse to go ahead and eliminate the rear springs and upgrade to a set of E-body HD's and with a set of B-body shock plates you can put in a rear sway bay at the same time.
Some folks on here like the polyurethane k-frame mounts. While I'm not disparaging the choice, the car is serviced better, in my opinion, with solid mounts.
Tighten everything up with a set of frame connectors.
Take a look at Firm Feel's website and explore what they have to offer. Take a look at PST and have. If you plan on going the B engine, you'll definitely want to check out engine-swaps.com
Just remember, much like building trucks, it's all a package. Horsepower and torque demand to put to the ground, so explore your options on how to best get there, weighed against your goals, and your budget.