There is no such thing as a converter that will not balloon. Under the right circumstances, the best converter in the world will balloon. I never buy anything from anyone that tells me their parts won't malfunction.
That being said, a converter does not have to balloon so big that there is no free space between it and the flywheel. It need only expand enough that it takes up some of the clearance that is necessary.
A converter need not balloon at all in order to cause thrust bearing failure. I have seen high stall converters that have poor low speed driving characteristics generate enough internal pressure to force the converter forward out of the pump with enough force to wipe out a thrust bearing in 100 miles, to the point of the crank hitting the block and the flywheel being bent. I measured the converter at a shop that builds them, and it was not ballooned. That same converter damaged another engine, while in front of a different transmission, in a different car.
There are a couple of other problems in the engine that can cause it, the most common being a crankshaft with taper on the journals. I've seen cranks with as much as 0.0005" taper on the journals, and if the taper is in the right direction, oil pressure will combine with the pressure from the converter and wipe the bearing out.
It is possible for the transmission to cause this extreme pressure also. It does not just happen to the 400 turbo either, it also happens with the 200 4R. I witnessed Jasper replace a 4.1 Buick V6 5 times for a destroyed thrust bearing. The car belonged to a friend and co-workers uncle, and he had in fact approached me about doing the job, I'm glad I was busy. After the 4th engine Jasper replaced the converter, and the 5th engine suffered the same fate. A new transmission was installed, and the next engine is still running.
The problem is brought on by several things. First, if you talk to 100 transmission builders, you'll only find one or two of them who replace stator bushings in the pump. Leaking stator bushings can cause excessive converter fill pressure. The other problem is worn valves and valve bores in the pump. Especially in the 400, this causes extreme pressures in the converter. While some will put a restrictor in the converter fill circuit, I ream the pump bore and replace the valve. Sonnax makes a kit for just this reason. I just do not like to restrict fluid flow into the converter.
