C4 Bell Housing

I have a 66 C4 auto and a 200 I6. I am no expert on transmissions.
I'm having trouble shifting after a rebuild.

I'm looking at getting a C4 from a 68 and a 302. Can I swap out the bell housing and use the 68 C4 on my 200 I6 ?

Also would it be possible to swap out the valve body. Which could help me get away from the green dot 66?

Welcome! I think you can but here's a good website that might tell you for sure:

http://www.mustangcentral.net/tech/c4.html

I got a C4 from behind a six for mine so didn't have to worry about that.