If you are doing full penetration welds all the way around each joint, I'd start thinking of methods for and gathering the items required to straighten the frames once they are welded. You will likley have some distortion in the tubes that should be able to be tweaked back to straight as required.
Assuming you are using Mig, I'd definately start with the gap as Geezer noted. If you put tacks at each corner of each tube, bridging the gap, it shouldn't shrink much more with the full weld, and the shrinkage should be uniform.
Try to keep your welds symmetric, rather than allowing all of teh shrinkage on one side, then trying to deal with it when you flip.
I'd make up a few test joints with the miters to get the technique down. You can cut and recut apart the miters on the test part time and time again to keep practicing and perfecting the technique.
Shoptask 3-in-1 (not currently in my garage, but I own it...)
Any day on a motorcycle like this that ends just needing parts and labor is a good day.
4.95, 154mph 1/8th mile 7.49, 194.27mph 1/4 mile