Here's my take.
I struggled and struggled to keep a 200 in my grey car that would live. After going through 7 rebuilds with 3 different companies (back in MS mind you) and 2 correct BRF cores, I finally got fed up. When the opportunity arose to make the switch I went for it. You will need to modify your tranny cross member or get one of the conversion mounts, different shifter and a new fill tube. The 400 that I had was fully rollerized and had a 1st gear ratio that was as close to the 1st gear that a 200 has. Some people said I would loose hp, but I didn't see it. On the last pass I made with my 200, the car went a 10.95 @ 124. Made the swap and the very first pass with the 400 the car ran 10.92 @ 124. Hmmmm.
This car eventually got a 274 Stage II and was setup to run in TSE class which I did for 5 years. (Eventually running a 10.07 @ 135) 4 of which were with the same 400 trans. And in those 4 years, I was pretty brutal on the trans and only had it freshened up once. When they say that a 400 is bullet proof at hp levels below 1000, they aren't kidding. I had a malfunctioning shifter that caused me to launch in 2nd gear on two different occasions. No damage. I did miss having 4th gear for cruising, but the car had pretty much gotten to the point that it was getting pretty hairy on the streets anyway. I would highly recommend that you have the tranny modified to accept a bolt on JW bell housing vs the standard BOP housing. Much stronger and a million times easier to access the bell housing bolts. I also ran a TCI scatter shield as well as a trans brake.
If it's a cruiser and occasional track visits, a built 200 is great. If it's a track only race car, or if you don't mind not having 4th gear and want strength, get the 400.
HTH
Patrick