Theres sites i have seen that talk about how to port one, and which head castings are best for performance, as well as limitations based on runner design because of pushrod location. Every little bit helps.
The problem with the 225 slant stems from the earlier design. Before the 225 was thought up chrysler had the 170 , and 190 slants. It was closer to a square engine. Small bore short throw crank. These were small inch but rev happy for a 6. Being that the bore spacing was already set, machining tooling was aleady on the assembly line, and the need to come up with a bigger engine popped up. Chrysler raised the deck height, and designed a different crank to go in the new block. This resulted in the same small bores with a longer stroke. Not the best in terms of performance.
My recommendation is to find either a 170, or a 190 slant, figure out which head to run. I believe slanty heads can be milled safely to .100", .120" to bring up the compression. In this case you will have to lop off about 1/8" of the bottom of all the head bolts so they dont bottom out before they torque the head down tightly.
I had a 69 dart slanty i did a head gasket job for somebody years ago. Took .100" off the head to get the warp out of it. Ended up cutting 1/8" off the ends of all the head bolts so they wouldent bottom out.
I could be wrong with some of my recommendations, however i am not wrong on the head milling and headbolts.
