i'm sure the m42 head has round exhaust ports and a square 4 bolt flange per port to hold it to the head (although i think most flanges only use 3 bolts as the header gets in the way of the 4th hole, as for positioning i dont think there would be enough room to top mount the turbo especilaly with the coil packs were they are in the oem position although you could fit a COP conversion plate to gain some mor space.
still i think by the time youve made a adaptor plate to mount the manifold you would be to close to the chassic rail to do a bottom turbo build.
Ant did a bottom turbo build on a M40 and had to put the turbo near the front of the engine were the A/C compressor would go to miss the engine mount, i'd guess you would run in to the same problem..
Not much space..

You should never underestimate the predictability of stupidity
M42 Supercharged 285bhp + M3 6speed box