Which trans? SPID & Build Sheet Each Use Different Codes

[Jahloha]

My truck is a 1970 C20 350

I am trying to plan out my project and trying to learn the pros and cons of my transmission, but only thing is that my SPID has code M49 which would be either for the TH350 or TH400...but my Build Sheet has code M38 which is only the TH350.

Is this normal?

I am building a dd and do not believe I have the skills to conduct a tranny swap, so I am wanting to pin point which trans I have, in order to read what all you have to say about the pros and cons of it. Or what I need to do in order to make whichever trans it is, a better dd (mpg).

Thanks for all your help with this.