For a nice cruiser/driver as you you stated you wanted, I'd leave the six in there. There are a few parts for it, but might be hard to find. The block is essentially a Chevy 230 six block (66-67, 260 for 68-69), but there were some changes to the casting in the front. IIRC it only uses the crank and rods from a Chevy six (and associated parts like bearings). I'm not sure about the pistons, probably specific to the OHC configuration. Find a 4V intake if you want to pep it up a little. Standard was rated 165 hp, the "Sprint" 4V version was rated 207 hp. There is a forum specifically for these engines (Pontiac Overhead Cam SIX Forum Forums
)!! Yeah, I'd say it's a darn nice car, and would be a shame to do a lot of mods that can't be returned back to stock if desired later. The four door hardtop is a cool style. You could shave the outside door handles on the rear doors to really trick people! Okay, shaving the door handles is a bit more than "easily returned to stock" since welding is involved, but door handle holes can be drilled back easy enough... You just wouldn't be able to get in the back seats from outside, use electric door poppers on the inside.
I wouldn't go with a roll bar. Those cars have a full frame. This site has a good photo of the frame with a tubular brace in the center (scroll down): Frame strengthening needed? - Chevelle Tech
Most sites say do nothing to the frame unless you're running over 500 hp. Same frame was used whether the car got a Pontiac 400 (389 in 67?) or OHC six. No extra bracing is needed.