Rule #1, when you don't have a lot of your own money, try to spend as much of other peoples' money as you can.

Engines are a lot like buying a new car. It's better to let someone else eat the cost of depreciation. If you look around you're going to find lot of engines that someone sunk thousands of dollars into rebuilding and then ended up not moving forward with their project, resulting in them selling the engine for a fraction of what they sunk into it. That's the engine you want to buy.
Rule #2, always buy the biggest engine because that's where you're eventually going to end up. I've lost count of how many guys I've known who've wasted resources buying, modifying, or rebuilding intermediate engines like 340s, 360s, 383s, and 400s only to end up taking a loss when they eventually decide they want a 440, which is where they always end up after taking their cars to countless cruise ins and hearing "too bad that doesn't have a 440 in it!" over and over again.

I would sell the 400 to someone who wants to stroke it and use that money to put into a 440.
Rule #3, not all 440s are the same. Down here in Florida, where RVs come to die, there is a plethora of low-compression motor home and truck 440s that sell for peanuts, which from a performance perspective is about what they are worth. While it's appealing to buy a "was running when pulled" motorhome 440 for $300 or so, you're going to violate Rule #1 when you have to find new heads, a better cam, intake, and have the engine rebuilt to get some semblance of performance out of it. Better to use Rule #1 and buy one that was built up for racing in a project car that's been cancelled.