[Sirrotica]

One other question, how many miles on the original engine? That originally came with a nylon timing gear in it, and if it is still in there the cam timing may be way off of spec. Retarding cam timing kills low end, so it may be sluggish because of worn timing components. Or you may have cam lobes going away, one or two nearly flat cam lobes pretty much kill power.

For sake of comparison I had a 68, 400 4 bbl Bonneville with the same gearing, got over 17 MPG on a trip, and would burn the single wheel as long as you wanted to hold your foot into it. Stock cam, and single exhaust.

I wouldn't go with 670 heads with the closed chambers, they have a tendency to detonate when running pump gas. If it were me I'd say with the lower compression heads with small valves. You're never going to be in the RPM range to make good use of larger valves anyway. The 389s used the small valves in them for many years in GTOs and most 4bbl engines previous to 1967. Unless you're building this car to be a street racer no need to build the engine like a drag engine is built, on the street you won't have much occasion to get in the 5000 RPM range very much.

Freeing up the exhaust, optimizing the timing curve, richening the jets up a few sizes, and mostly factory parts should get you some stump pulling torque to pull that heavy barge around.

I used a 1973 400 2 bbl engine with a 067 camshaft, and swapped to a Q jet and factory intake to pull around a 5800 lb Jeep truck. Low compression and small valve heads had more than enough power. Before I put the 400 in, I had a 350 Pontiac engine in it but broke a piston, and ended up swapping to the 400, because I had it already to go. Optimizing what you already have should be plenty to move that wagon around at a satisfactory pace.

I almost forgot, I had 2 of the 71-76 clamshell tailgate wagons with 455s in them, low compression, and I wasn't ashamed of how either of those wagons ran. I just optimized the stock parts on both of them, they are much heavier than a 67 wagon is by about 800 lbs, they weighed 5300 lbs.

Pontiac engineers excelled at making low RPM torque to pull their rather heavy cars around, and still keep the RPM down to get at least decent fuel mileage. Wagons have a higher percentage of rear wheel weight by the body construction, so if it won't spin the rear wheels, that's because you have more weight over them than a coupe has, figure accordingly.