Ever hear the term "size doesn't matter"? That doesn't pertain to engines!
400's aren't always easy to come by, which is what makes the 383 stroker motor popular. Without getting into particulars, it's an .030 bored 350 with a 400 stroke. Whether to bore the 400 or not is up to the condition of the block, it's done to true up the cylinders. The stroker and 400 has a bunch of low end torque (the "get up and go" you felt). Decide what your budget is, then see what kind of engine you can afford, mild or wild.

 

You can build a 350 to have plenty of get up and go also, but like Camro69 mentioned, there's no substitute for cubic inches. I wouldn't spend a ton of money to buy a 400 block, but if I stumbled across one I'd consider it.
Your TH350 will bolt up, but don't use the flexplate! The 400 is externally balanced, and uses it's own flexplate.

 

Oh there is a substitute as its name is forced induction. But in reality, no. The phrase "there is no replacement for displacement" still stands true. But then you start getting caught in another famous phrase. "cubic dollars"

 

The only part of that 400 worth keeping is the crank. The 400 is known for cooling problems with the Siamese cylinders and steam ports in the heads they are not the best motor for hot ridding. What males the 400 pull so nice is the long stroke crank. Putting the 400 crank into the 350 is in my opinion is the best day to go if you want the extra power.

 

I have seen some NASTY 406's. Scary fast cars. One was a 65 Chevelle that was in the 8's with a stock block. It was filled but still impressive.

 

i vote for the 400 i have stock block also never had any cooling isues yet,mines stroked to 436 cu.in.i love mine.makes great power and very well mannered idiles at 750 rpms i have even run it with a stock torque converter.

 

I must be doing something wrong. I can't get my puny little 327 with just .500" lift, and way less HP to idle at 750, or be mild mannered.

 

thats because ur running a carb and i was running fuel injection,a stealthram.but i hated the fuel injection but did idle nicely,its not gonna idle at 750 anymore probably,