225 performance upgrades without pulling engine

cam change.
2bbl manifold.
mopar elct ign conversion - if not done already. orange box.
headers will help HP but kill low end tq in S6..which it needs all it can get.
an easy to find carb is the carter BBD off of an early 318 ( i think they are 390cfm)..they can be found much easier than the similar factory 225+2 spec unit and fits super six linkage, man, etc.
factory 2bbl manifolds came in both alum & cast.
4bbl is overkill and can cause issues in a all stock 225.
In a rather low-pro motor like S6, taking weight out of the car can really help get it moving.