350 or 400 TURBO

Next step from my transmission post. I just found out that I have 350 trans in lieu of 400 like told when I bought the car. The 350 is in a 72SS bolted to a 70 396. The trans in the car is great and must have the mild kit as told because it shifts great and will bark the tires going from 1st to 2nd even at low acceleration. The reason for the 4OO question is that I am planning a 454 450 hp build soon and was told that the 350 would not hold up. I guess my question is will the 350 work or do I need to start my search for the 400. I have found one 68-400 with a 3500 stall converter but don't what it if I don't need it. As is said the trans now is great but maybe only for the 396.