The XR70 (and the Yamaha PW80) uses a swingarm with a different design to most; the forward end of the shock is quite far forward, and the angle is very shallow. The only way you'll get a different style of swingarm on the bike is to weld on a new upper shock mount that is closer to the rear wheel. If you were going to that trouble, perhaps it would be better to just lengthen the stock swingarm.
A shock in the stock location that is too long will push the rear end down and change the steering angle, which might make the steering a little too twitchy for your liking. The only way to combat that would be longer forks. Costs really begin to add up. Check out some of the fiddy sites; those guys put cubic dollars into their bikes.
I'm looking forward to watching this build!
150cc in an XR70 is going to be crazy fun.
BTW, chromed steel rims will turn into egg shapes after serious jumping; you might want to upgrade to aluminum rims. This is a good opportunity to master lacing and truing.