[Odd Job]

Is it the plastic inlet manifold fitted to your car, and is your car turbo or non turbo?
A very useful upgrade which I have done a few times on the 25 / 45 / ZR / ZS is fit the VVC style manifold which is aluminum. You need the one of a MGTF 135bhp. Silver one rather than the black one.
I've not fitted one on a 75, but I cant see it being any different.
You dont get the leaking water problem because the alloy manifold doesn't warp so easy.

As to your over heating, all the advice given above is spot on.
I'd go for a bleed first of all.
On the 25 / 45 they have a habit of circulating around the heater matrix and refuse to circulate around the radiator sometimes.
Air in just the wrong position wont allow flow through the rest of the system.