I did all the fabrication work under the supervision of my IA. The hopper removal is a must in my opinion, it requires removal of the spray plumbing along with removal of the hopper. Be careful removing the hopper and do not damage the roller pulley for the elevator that is the forward pulley at the end of the control shaft. You will see where the hopper sits in a saddle on either side of the fuselage. Mark below the saddle and cut that tank off leaving only the top and sides of the hopper top shell.
You may wish to fill the hopper top hole as you like, I used some filler board and fiberglass to complete this altercation. Now you can move below and start making a new bottom area or panel where the hopper dump was. This is really quiet simple and can be done by using some of the bottom pieces as a pattern. For your own safety I would use the hopper dump handle as the release arm for the towhook, you will not be sorry, just MHO. You can then go to a 337 from your IA if you wish.
A few other things to consider, make sure the AD for the wing attachment is completed, there were three, one is junk. Also the horz, attachment AD, this can be approved by an AMOC, that will follow the airplane as long as it is inspected annually and only used for tow purposes. Contact me through my private email and I can shoot you all the FAA contact information.