If you'll forgive me you're going about this the wrong way. Here's why.
I started with 2 pieces of box section as template shocks. I drilled them at 350mm and 370mm as I thought I would need ZRX shocks. My swinger is longer than yours and as such the shocks are more laid down.
I mucked around a lot until I decided that the front end was more important than the rear. When you have your engine in have a look at the bottom frame rail. This should be as level with the ground as you can make it. You will also want enough ground clearance to stop the exhaust scraping. I would suggest, as the J forks are long and you are changing to a 17" front wheel?, you need to get that right first.
At least you have some shocks to begin with and they can be the last thing on your mind.
See pic.
It's difficult to see here because I didn't take a side on pic, however, It became clear pretty quickly that the forks suffered mostly when changing from 19" to 17" wheel.
I decided on a 2" extension to the forks which are now dropped through the yokes by 1/2"
What I'm in essence saying is that a 17" rear wheel with larger tyre is not much different to the 18" you had to start with.
Leave it 'til last.
(yes both in picture are right hand, our Japanese friends messed this up but was all corrected in under one week)
