Yes, the converters will interchange, but the stall speed will be different on the 350 converter. Unless the 350 converter is the same diameter as the 400, the stall speed where the engine RPM stops increasing with the throttle floored and the brakes locked will be higher using the 350 converter. The smaller ther converter diameter, the higher the stall speed will be. There are some flex plates that have a dual bolt pattern so either 350 or 400 converters will bolt up.
Jerry

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