Will a v5-6 inlet manifold fit onto a ver3 at all ?.

 

Not without swapping to the v6 heads
And if you do the swap you almost certainly need a standalone ECU, as the sensors etc on the 2 heads are differnet (for example the idle control on a v5-6 is integrated into the throttle body, on a v3 it is a seperate item.

Also the swap itself seems a bit questionable, as the v5-6 manifold is not better in any way as the v3-4.
IMHO the V1-2 is best for top end power,as it has the largest collector area, the rest are al equal in performance (roughly put)