son wants to put ladder bars on the car was wondering what length shock every one is running

Coilovers?

Set the car up at the height he wants.. see what the
length is... most likely it will be a 14" total length
but can be different... there is a design length for
shocks so see if his car at height falls within that range

yes coil overs on an a body

A 3850 Afco (12" compressed, 17" extended), a Strange S5206 or S5006 (similar comp. and ext. numbers) or similar shock, installed at 13.75" - 14.50" is very common. 12" spring at the correct rate according to the rear weight. Get the car to the desired ride height and make the shock mounts the correct C-C dimension so the shock will be at the correct installed length. You could go longer on the shock. I would not go shorter on a rear shock. Big horsepower, an aggressive starting line gear ratio, manual trans and/or some other factors may make it a good idea to step up to a more aggressive valving on the shock.

thanks for the reply I have a set of QA1 that are 17 extended by 11.6 compressed so I think these should work

You can install an adjustable lower shock mount. Most ladder bar and four link brackets have one of the brackets drilled for adjustable shock mounts already.

Those QAs are the right length. If they are single adjustables with off the shelf valving and/or the car is quicker than mid to high 10s with ladder bars, you may outgrow them quickly. The adjustable mount makes it easy. Put the correct spring on it to maintain C-C on the shock at race weight and move the mount to get ride height where it needs to be. I would shoot for 13.5" - 14" max installed with ladder bars.

I've ben told to set the ride hieght on race cars ready to race with the driver in it at 1/3 up from fully compressed
you can use a Tyrap on the shaft at ride hieght and then see how much extension you get on a good track, go from there as long as it is not topping the shock travel out If it tops the shock travel out get longer ones or slow the extension speed down