Doug Dutra is the Slant 6 guy.
The four main engine has a number of disadvantages but one of the worst is that the bore spacing is uneven. It's 0.020" more between 2 and 3, 4 and 5.
The engine layout just BEGS for a turbo. All that room plus exhaust right into the open space. But the engine is only 225 cubic inches. It has a 4+ inch stroke already, so there is no easy way to increase engine displacement. If you are bound and determined to do this, look up Dutra online.
The head is a bottleneck. Ported you may see 200/150 cfm intake/exhaust.
Fuel distribution is a problem, port fuel injection solves that problem nicely.
The problem is money. You may have $3,000 to $5,000 tied up in a 300hp engine.
R.