If you want your trunk to be thicker, chopping now is the wrong thing to do.
THere are basically 2 ways to get a thicker trunk:
1. let the tree grow very tall.
2. Grow sacrifice branches from the base.
Your lowest branches might be low enough to work as sacrifice branches, but I think your best bet for this tree is to let the top grow. REALLY grow - no pruning at all - let it get 6 - 8 feet tall. That will give you a decent base. At that point, you do a trunk chop above one of the low branches which will become your new leader. Then you let that one grow tall, until it's width is just a little less than the base and you chop that branch back to a side branch that becomes the new leader again. That is how you get width, taper and movement into a trunk.
Brent Walston has a good article on this technique:
http://evergreengardenworks.com/trunks.htmAnd here's a video of one of his hundreds of JBP he has used these techniques on:
I drew a little diagram on your pic. The yellow line is your "sacrifice trunk" that you should let grow without pruning till it's 6-8 feet tall or more, depending on how thick you want the trunk to be. The red lines are low branches which should all be kept - one of them will eventually be your new leader, the rest will be sacrifice branches. The blue lines show small branches you can remove, to help keep the lower branches from being shaded out.
Good luck!
- bob