Swapping my transmission

Well as it turns out I have a FMX transmission - Always thought it was a C4. Well there doesnt seem to be a ton available for the aftermarket with the FMX, so I'll be switching it out for a C4

MY Question: Is there anything else I'll need to change out when doing this swap? Will my "teeth" be the same? How about the length of the axle? This will be bolting up to my 351W by the way. I just want to make sure I have all the info before I jump into this project. Any insight would be very helpful.

Well I think Im going to have more problems with it in the future. For one, it's a little sloppy - The dip stick for it, is going to be in the way of my long-tube headers. I cant get a rachet shifter for this model (Havent found any yet that is) Havent had much luck with the stall either. Figured with the prices of C4's being so cheap, why not just take care of this, before it makes me spend more moeny in the long run.

My main question though is - will i have to change anything else with the swap? Axle? Are the teeth the same?