The V6 was available with MT75 4x4, but it is a different gearbox (bell housing and input shaft) then the 2.0i box.


The 2.9 12v has holes in the top side of the block where the tappets sit, which the BOA doesn't have. Besides that you would need both cylinder heads, cams, the plenum, the exhaust manifolds, pistons and crank. So much easier to just get a whole BOA engine in that case.


When converting the 4.0 with BOA heads you have the same problem with the tappets as on the 2.9. Besides that the block is taller and wider so the timing chain and sump don't fit. It has been done though in 1 or 2 cases, but I think it was not very reliable. I'm not sure if even the standard 4.0 fits the Sierra, the engine (especially the plenum) is a bit higher than the 2.9.