As far as I'm aware and I also asked on mlr and most seem the say that all Mitsi manifolds are the same diameter.
Also spoke with my mate who has many years experiance in race engines and port Ian's polish etc and mentioned to him what you said about the bore of the mani to be the same diameter 70mm right the way through its length and he said no not essential as by the time the air has passed into the first inlet port then there is then 25% less air so no the bore size does not need to be 70mm right the way through.
I also noticed with some after market ones that they taper to a smaller size by the time they get to the last port anyway so just enlarging the tb port to 70mm is sufficient.
Also the 70mm hole should not be flat or square. It's should be tempered so as to encourage a ventury effect
