The fail in post response here amazes me ...
V8 is your best option hands down.
To do a m112, you'll have to upgrade to a split port. Which will also require that you swap then fuel system.
Forged internals are NOT "required" to run FI. Especially to beat a stock GT unless your speaking a new 5.0. Even then the stock internals on a proper tune can handle 350ish rwhp ...
OP if you want to go SC on this car. Look into an M90 swap w/ a super coupe motor.
Otherwise a Turbo will be your smartest choice for power.
Btw, to those unknowing, a single port turbo V6 I believe still holds the 1/4 mile V6 record. Oviously built motor on high psi etc etc so dont think Im saying you'll get close to him on a stock motor.
But honestly in the end, unless your just dead set on having a "built" V6, and have the money to do so, then find a nice V8 of some sort.
227whp N/A V6
1/8th mile 8's !