70 Wildcat Brakes: in great shape after recent freshening, but fronts lock first

I replaced all wheel cylinders, cleaned and painted everything else (including backing plates), reinstalled and tested the car. All shoes and drums were in good shape, so I cleaned and reinstalled them too. After all this work, it has no leaks and good, firm pedal. But I did a quick stop in gravel beside the road, and locked up the front brakes. Walking behind the car afterwards, I noticed that there were tire treads, from the rear wheels, in the skid marks! In other words, the rear wheels were still spinning as the car moved over the gravel, even when the fronts were locked. I'm assuming this inequality in braking power is why the braking on this car is less than stellar (although adequate for all but the most panicked stops), even for four wheel drums.

The rear brakes are definitely getting at least some braking action (with the axle off the ground, no problems holding the wheels still, engine idling in gear). As far as I know, original MC and booster. Did GM really do such a poor job on this car's brakes? I think not... what am I missing? Shouldn't I be able to lock all wheels on gravel?