What would be the advantage if any of using a 400 automatic over a 350 automatic is there any? I have had and seen some very screaming 350's out there and was wondering if there would be any benefit of changing?

I have either available to me and was wondering which on to stick in my 72 Monte Carlo?  

Depends on how much Power you plan to apply. 400 is a much stronger trans and can be built to handle more power than the 350. But, the 400 is heavier/the Trans & Internals and it takes more power just to turn. Also the output splines are different = driveshaft Mods. and different crossmembers.
 My Personal Preferrence is a 2004R it will bolt right in No Driveshaft or CrossMember Mods. Can be built to handle plenty of power.
Gear ratios of the 200-4R are: 2.74 first, 1.57 second, 1.00 third, .67 fourth!
1st,2nd,3rd at the DragStrip. Say a 3.73Rear Gear then on the ride back home the .67 Overdrive with lockup converter turns those 3.73s in to 2.4991 or 2.5 Highway gears SWEET
 Some prefer the 700R4, But Again Driveshaft and Crossmember Mods. Also I dont like th gear ratios in the 700 3.06 1st gear, 1.63 2nd gear, 1.00 3rd gear,
.070 4th gear
Theres too much rpm drop from 1st to 2nd, 4th 30% reduction while the 200 is 33%

If I'm incorrect someone will let me know. Welcome    

I HAVE A 92 CAPRICE TOOK OUT THE STOCH 305 AND 4L60(700-R) TRANNY PUT IN A 350 PLUS 350 TURBO AND LOVE ITCOURSE IM UNDER 400HP NOW IF YOUR GOING TO GO HIGHER THEN THAT ID GO FOR THE 400, BUT WITH A TH350 THAT IS GOOD AND SOLID PUT A SHIFT KIT IN IT AND YOU'LL BE FINE