* Shock type: Are they air shocks, air springs, or standard hydraulic shocks? Air shocks and air springs are adjustable; standard hydraulics are not. You can't adjust the pressure on standard shocks.
* Vehicle load: How much weight is regularly carried in the El Camino (passengers, cargo)? More weight requires higher pressure (for air shocks/springs).
* Desired ride height and stiffness: Do you prefer a firmer, more controlled ride or a softer, more comfortable one? Higher pressure generally results in a firmer ride and higher ride height.
* Shock condition: Worn-out shocks will affect the optimal pressure, even with air shocks.
If you have air shocks or air springs: The manufacturer's instructions (if you can locate them) will provide a pressure range. Start with the lowest recommended pressure and adjust upwards to achieve your desired ride height and firmness. Do this incrementally, checking the ride after each adjustment.
If you have standard hydraulic shocks: You cannot adjust the pressure. If you're having problems with the rear suspension (sagging, bouncing), the problem is likely worn shocks that need replacing, not air pressure adjustment.
In short, without knowing the type of rear suspension your El Camino has, there is no answer to your question. Check your owner's manual (if you have it) or consult a mechanic specializing in classic cars.
