A lot of talk about parasitic drag on a T 400 as compared to a T 350, unless you're running a 10 second or quicker combo drag car, the parasitic drag is negligible between the two. That thought is a quote from Jake himself on his website talking about 4L80E parasitic drag as compared to lighter duty OD transmissions used for performance rebuilds. A 4L80E uses 75% of the same internals as a T 400 has so the comparison is a valid one.
A street car is never going to need the lighter internals to see any difference, lighter internals also have less strength for the same materials used.
Does anyone find it strange that Cliff Ruggles runs a T 400 in his own street car as opposed to a T 350, even though he has written a book about the T 350?
My money is on a T 400 or a 4L80E (basically T 400 internals with overdrive capability) for longevity. The extra beef will have no noticeable effect on performance, but will withstand abuse over the long haul more effectively.
One more point is, to get the lighter duty transmissions to live in a performance environment the hard parts in most cases, need to be replaced with better materials. The stock factory hard parts in a T 400, 4L80E, will live just fine to 750 HP, giving your project room to grow as far as future improvements in your engine program.
There is a reason that the T400 and 4L80E transmissions weigh more to begin with, they have more mass to take abuse from the initial design. They don't need the band aids that the aftermarket has designed to try to make them live in an abusive environment.
I like to do a job one time and not keep re-doing it because the original design was deficient, and the weak link in the chain was uncovered. Spending my adult life in vehicle, stock car racing, and shop maintenance areas, has taught me to remove as many weak links in a mechanical device as possible, and never have to revisit it for breakage. JMO............
Other than a slight weight penalty, a stronger transmission platform has no drawbacks that are evident to me.........
That said, the OP seems 99% committed to the T350, but he has asked for opinions, and I have stated mine, FWIW.