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I just noticed that the torque converter on my th350 is an ACC Boss Hog #46033---which is listed as a th400 t/c. Is there an advantage to running a 400 t/c on a th350? Original owner said tranny was built by a performance shop. I know that some run a 350 t/c on a 400 to get a higher stall, but I hadn't heard of them being swapped this way.

TH350 and TH400 both take the same converter. An OEM converter from a TH400(only originally available on high perf. and heavy duty v8 applications) might be a little tighter than one originally from in front of a light duty 4, 6 or small V8 TH350. Anything aftermarket would be meant for both.
And like the majority of drag race Powerglide transmissions, mine have been modified with aftermarket input shafts that let me use a TH350/400 converter.

Thanks for the reply Spencer! I thought they were interchangeable but just wanted to make sure. Still not sure why he chose the 400 t/c rather than the 350 t/c, same specs except outer bolt pattern size, maybe had it sitting around the shop?

I forgot about the different bolt patterns; most GM and all aftermarket flexplates come with both sets of holes.

Usually an advantage in t/c is measured in higher stall speed. Stock t/c s are larger in diameter .Higher stall speed t/c s are usually smaller at 10,9,8 inches in diameter. Depending on flexplate bolt pattern , a 350th and 400th can use the same t/c.

The 8" FTI converter in my drag car stalls at 5500 rpm on the trans brake behind the almost 600 hp 383. That's past the red line of the majority of SBC's in stock form. Back in the day, OEM 4 cyl Vega converters were highly sought after because they would stall 3000+ rpm behind a healthy SBC. Yeah, I have one on the shelf, but I wouldn't dream of trying it out. I suspect it would last maybe 2 or 3 passes.