1965 valiant /6 to 340 a833 and 8 3/4 rear swap

Yes you will need the correct motor mount for an early A to a 340. It does work best if you can get some early A pedals from a donor. The trans cross member should work ok but you will need a drop center link.
I do have a picture of the 2 drop center links I could post when I get in.
The Z bar in an early A is shorter than other cars and the fork is different on the early cars. I believe I bought my 4sp parts from Brewers.

As far as shorting the driveshaft it will depend on weather you go with an 8 3/4 or 7 1/4 and if the trans has a conventional U joint or not.