The main issue is how a bell housing mates to the new V6, clearing the
flywheel and handling the starter. Years ago I stuffed a Holden 186 into a
Commer 2500 van, replacing the anaemic 1.7L 4cyl.  The original Commer bell
housing mated to the gearbox over a circular plate containing the spigot
bearing.

I got a 186 autobox torque converter housing (ie the front half of the bell
hosuing) which was also circular, bolting to the rear half (ie the Holden
auto trans). The Commer bell housing was cut approx in half, and an
aluminium ring welded to it. All that was now required was to maching this
ring to fit the torque converter housing such that the spigot shaft fit into
bush in the rear of the crank. I needed a suitable bracket for the clutch
slave cylinder, in this case made from a piece of 3/8 plate bolted across
the bell housing behind the flywheel.  Not particularly difficult, just
tedious to get the distances right.

If the Holden or Ford V6 still uses a torque converter housing (I live in
Canada now) you could likely do something similar.

SD