Minimalist Slant Six draw-through build...

I’ll add three things.
1. Stud the deck, anything you can do to a slant to improve head gasket sealing will help.
2. In this day and age a draw through setup is archaic at best. They really do not work well and are a bear to tune. Especially for drivability. Consider a blow through setup with a modern carb (CSU or similar) or just skip the hassle and go EFI.
3. Do not lock out the timing. Unless this is a race ONLY car. Run some type of timing control (MSD 6btm or similar) that allows you to keep a normal curve and pull timing based on boost.