You fail to think about the fluid passages needed for the secondary valvebody to be functional. Even if you did install the C5 valvebody and the torque converter (assuming it fits properly)...I'm 95% sure that the only thing you'd end up with is a poor shifting c4 without a lockup converter.
I just rebuilt my kid's 83 BII C5 and it was a non-lockup converter but has the timing valve body. From what I gathered there's an extra set of splines at the end of the input shaft for the C5 transmissions that use a lockup converter and they were used in passenger cars. I haven't seen this to validate that info, so I'm passing it along as I got it from a tranny guy I asked. By the way, I did this as a conversion to get rid of the lousy A4LD in his 85 BII. It was a lot of work, but it was worth the reliability.
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