How much extra HP can I realize from adding a cam and 4bbl to slant 6.

I have a '65 dart 270 with the original 225 c.i. slant 6. I want to replace my original 1bbl Holley with a 4bbl, 650 cfm Holley and intake manifold and new exhaust headers. I also will add an aggressive cam and bore out the the cylinders .030. The stock motor makes 145 HP. It's quick, but I need more HP. I'm torn between modifying the tower of power slant 6 or changing to the 318 c.i. crate motor. The 318 makes 300 BHP, but I'm not sure I want to change everything for it. If I could boost my slant six to over 225 BHP with the upgrades that I mentioned I would stay with the bullet proof slant 6. My 225 c.i., slant 6 runs great and only has 44,000 miles on it. I hate to change it out, but I need over 200 BHP to satisfy my need for speed. If you know how to add over 55 HP to my slant 6 please eddify me. Truely, I would love to find an upgrade of 80 BHP to the 225 c.i., but I don't if I can get there. If you know how to get over 55 to 80 BHP, please let me know. I really don't want to go to the 318 c.i. because I'll have to change the transmission. Expensive!