Bayani, easier than reworking the handbrake box is using a pair of metal strips between seat base and adjusting tracks to offset the seat base sideways toward the door. Not only does it fix the handbrake problem, it makes the firewall bulge less of an issue. Sure it twists the passenger a little on the seat, but thats not so much of an issue as with the driver who has to deal with pedals & a wheel. I did this to our '72 some 15 years ago when I had our stock seats rebuilt & modified. And since Judy & I trade off driving so I spend time in the passenger side, I can verify the offset does nothing serious to seating comfort.
I used 1/4" thick aluminum strips 3" wide & moved the seat 1-1/2" outboard; if I were to do it again, I would angle the seat on the strips rather than keep it perfectly parallel, aesthetics aside.