Building a slant for torque
I'm always curious as to why people still even build those things.....
I like something different. I do not buy into the "just throw a LS in" school of thought. Yes they make torque and thus HP. But a lot of modern engines do also. The LS are probably less expensive.
Now a slant is probably original to the car. There is nothing else like the sound of an inline six with dual exhaust.
I would consider a replication of the Hyper Pac for the slant. 10:1 or 10.5:1 pistons, port the head with a bit bigger valves, a bit of a cam, the repop intake and an Edelbrock 500CFM AVS2 carb and a 6 into 2 exhaust header.
When porting the head, I would seriously consider 40° exhaust seats to aid low lift flow with restrictive ports. 225's are not a high rev engine for the most part, so I would be looking at 1.5" primary header tubes. You want the 4th resonant low pressure arriving at the almost closed exhaust valve, to create a low pressure in the cylinder/combustion chamber at the time the intake opens.
There are also resonant frequencies in the intake ports. Tuning exhaust length and collector length can get these to work together, to enhance torque or broaden the torque curve.
Look to UTG for inspiration. For street use a slant can be a fun driver and get good fuel economy.
For a less expensive intake, search for a Super Six 2 barrel intake that was factory installed.