You said you had a 4-speed; I assume that 4-speed has a 2.66 low gear;
have you idled it down to 500 and tried to drive it there.
If you did and it didn't buck, then
1) it wasn't Idle timed in the usual place (15 to 25 deg). and/or
2) your 298/308 cam is not rated in the usual way.
Even a 220*@.050 cam will buck at 500/550 with the usual 15 to 25 degrees of Idle Timing...... because the transfer slot sync is in the wrong place.
The only other way that it won't buck, is if the cylinder pressure is in the basement, and then the thing will be embarrassingly low-powered from idle to 3000 rpm or more, which you didn't mention.
