Gear issue or too tall tires?

[rico7769]

Team,

I am running a 350/400 setup in our truck. I am not sure about the gear ratio but it is whatever is stock for a 67 GMC short bed with the CST pkg with the original motor being the 292.
When we drive the truck, it feels like our gear setup is approx to the following numbers...
1st- 0-10
2nd - 10-25
3rd-25-65 (around 3500rpm @ 65mph)

We are running 255/70/15's all around and cannot get over the lack of power it seems to provide with the motor we run on. 350(decked), aftermarket cam, performance heads, Holley 600, Accel ignition/wires, headers, etc. Feels like we are pulling about 5000lbs behind the truck at all times but will still get sideways if you get on it. Seems overall like we're missing out with the gear combination or do you guys suggest a change in tire size to accomodate?