Horsepower rating for the 225 with a 2 barrel carb was 160 HP. The 1975 231 was rated at 110 HP. The rating procedure may have changed during that time. I think a late model 1980 or newer 231 has more power than a 225 due to a better head design, and will make a lot more power when modified. The high port 231 heads will bolt to a 225, but according to Pat Ganahl's V6 Performance book, water leaks are possible. If you used a 3.625 crank, you would have to change the engine over to even fire and use custom pistons, unless you found a odd fire stroker crank. That means the cam, damper,and distributor would also have to be changed to even fire. I think there is a difference in cam bearing diameter also. You could also just port and mill the 225 heads, change the cam, intake, carb, and get a set of headers all made for the early engine. A complete engine change to a 4.1 would be a much cheaper and better option, and then you could modify from there. After you do all that work and spend all that cash on the N/A V6, a stock V8 will have as much power.