Anything is possible, but save yourself a lot of money and hassle and stick
the Mi16 engine in.  You can buy the parts off the shelf, the engine will
cost you peanuts and it'll fit straight in with the minimum of hassle.  To
fit the V6 you'll probably need lots of parts fabricated (exhuasts,
manifolds, radiator, engine mounts, driveshafts etc etc), then you have to
try and get it in the engine bay, wire up all the electrics, find a gearbox
that will fit and lots more.

Then think about driveability.  I'd imagine the V6 is fairly torquey low
down, and in a light car like the 205 that probably means lots of wheelspin.
You can sort the Mi16 so it is more 'revvy', which would suit the car a lot
better IMHO as it would allow it to build up the power and reduce traction
issues.

Just my 2p anyway.