I guess it depends on what you want out of the wagon when you are through. That 'lump' of a 350 is a good solid engine, and built right will give you more than adequate power and torque for a daily driver, but it is not likely to set the world on fire in terms of off the line performance. It will, though, be able to give you more than reasonable fuel economy and still be able to get out of its own way quite respectably. If it were mine, I would get the Edelbrock heads, intake and carb and what ever cam they recommend for torque, and build it up that way. Add an over drive transmission and the fuel economy will be very much acceptable, and for a reasonable amount of money, too. After all, in the mid-70s, that 170 horse lump was moving 5,000 pounds of big Poncho and doing it with relative ease. Built properly, say about 225 horse, and you will not notice a significant power deficit, in real world driving at all. It is still more than the original engine likely had anyway.
The Tempest wagon weighs in at a very respectable 3,340...a smidge under the GTO at 3,445, 3,455 and 3,555 (coupe, hardtop, and vert). It's also 3 inches shorter and my particular honey is the lowest prod run of the year @ 4095. That being said, no it's not a GTO, and I'm not gonna be a weirdo and put GTO badges on it. BUT, it will be a bad mofo with a 450 hp+ engine, 6 speed (TH400 +Gear Vendors), 4 wheel disks, monster suspension, and just general badness. No it won't be a daily, weekend terrorist is more my mode.
Scary, with that weight, build for torque and let the gears get you the speed. Any Pontiac engine is going to give you great torque anyway. What ever you do, you'll be having fun with her!