3.4 V6 to 350 V8 swap - Speedo too?

[Derek up North]

I'm not sure if this is the right section of the Forum to post this question in, but here goes.

In a 3.4 to 350 swap, can the 3.4 instruments be kept or will it take the instruments from a V8 car to get the tach (and speedo?) to read correctly?

Thanks in advance.