Help me STOP

[Burbinite]

I have a '72 GMC 4x4 suburban with stock brakes. Disc front, drum rear, power brakes. (this is my first 67-72...anything) It sits on 33x12.5x15s. I have to use a lot of strength to stop this truck quickly. When I pull a trailer without brakes it's a both feet on the brake pedal, push as hard as I can, with a quick prayer to get the thing to stop in a decent distance. Even with an empty trailer. (I'm not putting it back on the road until this is fixed)

Here's my question(s): is this normal? Is it really this hard for the stock brakes to stop this truck? Is it the oversized tires? If the answer is "no" I will replace brake components with stock replacements and hope that fixes the problem. If the answer is "yes" I will put an upgraded brake kit (baer or wilwood) that will stop this thing quickly and safely.

Any advise is greatly appreciated.