DR400/500 & SP500 - 393 pivot to shock mount
80-82 PE250 & 400 - 534 pivot to centre of axle slot and from pivot to shock mount 330 +/- mm
79-80 RM250 & 400 - same length as PE but the shocks mounts bit closer to pivot and have better clearance at lower mount for spring retainer and you don't need long eye mounts or require cutting the gusset like on the PE's
Keep in mind that shock mounts closer to the pivot will not always give you more travel. Only will if you can fit a long enough shock with enough stroke and also not have your swingarm/chain angled down too much. Sprocket sizes also help determine limits. Often having the shock mounts closer to the axle will allow a longer shock with more stroke to fit at the same swingarm angle with as another with shorter shocks with mounts closer to the pivot, therefore giving more travel. Unless you find someone else has done the exact same modifications beforehand and done all the work, you will have do your own research and experimentation to find out what will work best.
A lot of these s/arms will require some sort of modification to the front end as well to match the rear.