Classic/Antique Car Repair: Engine Miss, valve train components, comp check

Question
Ihave a 1970 Mustang with a 302 CI engine. Just all of a sudden the engine started to miss very badly so bad I couldn't drive it.
The first thing I did was a complete tune up: Plugs, plug wires, points, cond.,dist rotor, dist cap. This did not affect the  bad miss.  So the next thing Id was to run a comp. check on each cylinder and all readings were within specs.  Then I remembered  that a Jeep 6 cyl I had broke a spring on the way to a car club meeting and had a similiar miss.  So I pulled both valve covers for inspection and got a big surprie.  On the second cylinder on the driver's side, both the intake and exhaust rocker arms had slipped off the valve spring.  I  examined the springs, push rod and rocker arm and then put the rockers back in place and started the engine. I still have a small miss, but it is not to bad to drive the car.  
Now, for my question, I think I might have a weak spring on this cyclinder. Which would be best, to replace all springs and push rods or just replace the springs on the suspect cylinder. I don't want to have a valve job right the present maybe in a year or so.
Any opinions, suggestions, or any advise will be very much welcomed and appriciated.
Thank You
jerry

Answer
Hello Jerry,

I'd check one other component before you decide what to do: lifters. They are hydraulic on your 302 and are subject to wear, sticking and even collapse with age.  Rocker arms should not slip off the valve train as they should be held in tension by the lifter/push rod assembly. Your valve springs could indeed be weak also but I don't think they're the culprit in this particular problem.

So, check the lifters, bent or worn pushrods and check cracks or galling at the attachment points on the rockers. All of the information you provide paints a picture of a worn engine--you may be band-aiding things when you might just need to break open your piggy bank for a complete valve job including checks/replacement of the critical valve train components.

As far as should you replace one or all springs, rockers, lifters etc., I'm afraid that my experience shows that if you need to replace one cylinder's components on a high mileage engine, it's likely that the other cylinder's aren't too far behind. Keep your reliability high and frustration lower and replace all that you find defective.

Good luck on this great car!

Bob