if i remember right the shop that put my 799 heads on used 7.350" pushrods. replaced lifters with the ls7 lifters and the heads were milled .030" but that's it for what would matter. stock cam too in my lm7.

 

If you are using true ls6 heads with the light weight valves that pr is WAY too long. Their valves are .100 longer. Otherwise the difference in the ls7 lifter vs the ls1 isnt that much to necessitate a longer pushrod than 7.400.
Normally a 7.400 pushrod will get you in the window of preload for nearly any cam unless they are the stock ls6 cam or a regrind with untouched heads. If they are milled or you are using a thinner gasket then youll want to measure.

 

not the light weight valves any more in the heads, i got the car cammed with those pushrods already installed speed inc put the car together, i re-cammed the car and just reused them