I see four avenues.
#1 Build a stoke 10.1:1 440. The factory rating in 69' was 375hp/475tq. This will give you strong street performance with the driveabilty of a cruiser. Smooth, quiet, responsive and no maintenance. Add a few goodies like a intake and headers and you're over 400hp.

#2 stock displacement but racey. This moves the torque higher into the rpm range which results in more hp, but sacrifices responsiveness, smooth, quiet and driveability. Requires change in converter and possibly gears. Estinated $2-2.5k for good parts.

#3 Maximize displacement. Build the bottom end bigger, little to no changes up top. More torque, more responsive, smooth, quiet. 50 to 70 additional cubes will net an additional 100 ft lbs of torque give or take. And allows for additional cam duration and lift without sacrificing driveability. Estimated $2-3k in budget over stock parts.

#4 Build the top for additional air flow, stock bottom. Not as efficient as adding cubes, but will result in additional torque throughout the rpm range. Estimated $2k to $2.5k in budget over stock parts.

#5 Combination of #3 and #4. Stroker with better breathing top end. Greater torque throughout the rpm range. Great driveability, smooth, quiet, responsive and fast...very fast.

Molnar or 440source for the bottom. TF240 for the top. Edelbrock for the induction, Hughes for the valve train. Estimated additional additional $5-6K, plus machine work and assembly, in the budget.

Start with the budget. $3k?, $5k, $8k?