You can rev them to 4500 but for a stock 235 no damage will happen at 3800. I consider 3800 the max to rev to under normal everyday driving. A well built and balanced 235 will stand a bit more. The redline speed is a function of the rod ratio, the stroke, the reciprocating mass, and the materials. This adds up to a number at which the rods start to stretch because the pistons are too heavy for the acceleration produced. 235's and 261's have a 3.9" stroke, 292's have a 4.12" stroke, but 292's have better rods and 7 main bearings, in addition to a longer rod ratio (length of rod to length of stroke). Harmonic vibrations tend to kill the 235/261 faster than rod stretch will, because of the main bearing configuration and general inattention to balance. A well balanced 235 or 261 with forged rods and aluminum pistons could probably hit 6000 rpm without damage. The 250 has a 3.5" stroke and can stand sustained higher rpm easily.
Usually the horsepower peaks at 3800 in a 235/251 anyway. The head can't support much more flow.
