1. Any electronic speedometer from Autometer will be perfectly fine.
2. On our personal car we are running a 25x12x4 V-mount setup with 3" piping, but customer cars run a 25x12x3 all day long with 2.5" piping
3. No issues with any aftermarket shifters, from no-name brand B&M style to the C's style short shifters as well.