Can you tell the difference between the sounds of ignition knock, bearing knock. or piston slap? This would be a clue of where to look.

By your description, this knock appears when extra torque is produced. (The knock sensor can not tell the difference between piston slap or ignition knock. It senses the high frequency shock waves in the head or metal on which it is mounted. In fact, one way to test the knock sensor is to tap the head near the sensor with a hammer to see if it retards the ignition.)

At 300,000 miles, you may have a worn out engine. It could be a rebuilt, but you don't know how many miles ago. A worn piston will slap under load and this decreases when the piston is hot. The piston temperature will vary while operating. It gets hotter with increased loading.

You could have carbon build-up in the combustion chambers that causes ignition knock under increased load.

A likely candidate is worn main bearings (crank shaft). A worn crank bearing on either end of the crank may cause the seals to leak.

The ignition should retard a bit with falling vaccum such as when under high acceleration down a freeway ramp. If it does not, knocking can occur. With old style engines, this was accomplished with diaphragm device on the distributor. On newer engines, the manifold pressure is measured with a sensor and its signal used by the electronic system to set ignition advance. A failure in this system can cause bad ignition timing.

Another possibility is oil starvation but since you changed the oil pump, I'd expect the usual causes for oil starvation would have been corrected by you at that time: Items such as clogging pickup screen on the oil pump or trash in the oil pan that may get sucked onto the screen. Besides, oil starvation will cause rapid wear of affected parts and early failure. Apparently, you have been able to operate this engine for some time with this condition.

Another possibility is ignition timing. I am not familar with your engine and where the ignition system gets its timing signal. If you engine is of the old type where the timing is manually set off marks on the vibration damper, be aware that these marks are on the vibration damper ring and if the elastomer between the ring and hub debonds, the damper can slip carrying the timing marks out of place. To check for this, remove no 1 spark plug and feel for the piston to arrive at top dead center. Do this carefully. you do not want to smash something betweent he piston top and the head. Park the piston at top dead center (TDC) and then look at the timing marks. These should show TDC. If these are not TDC, obviously the marks can not be used to time the ignition. I have seen ignition timing set ahead or behind as much as 15 deg when the damper ring slipped.

Also, check the paint color of the damper and the engine top. It may be either red or blue. The damper color should match the engine color. The accessories may be on the right side or left side of the engine depending on what vehicle model it is installed in. Red or blue paint was used to denote a right or left build. The timing mark bracket may be on either the right or left and the damper mark will be placed accordingly. In rare cases when refurbishing an old engine, a mismached damper can be installed. New marks should be put on a mismatched damper. I do not think this is your problem since the engine runs ok.