I think you will be fine. I'm 5' 10" , wear between an 18-20 on the bottom (depending on what kind of pants), but I have a much smaller chest than you.
We just got back from our trip. My anxiety levels were through the roof with this ride. My husband didn't travel with us so it was just me and our 6 year old daughter so there was no one to give my harness that extra push.
We were staying onsite so we had the extra hour in the morning without all the crowds. The anxiety got worse as we got closer to the castle. My daughter and I stopped at the outside test seats and I got a yellow light. The girl there said if you can sit like this for about 5 minutes you are good to go. Anxiety eased up a bit but it wasn't gone because I remembered sometimes you fit outside but not inside.
Got to the queue, walked to our bench, and grabbed an end seat on the left with my daughter to the right of me. I lucked out the TM who helped me was awesome. I sat, raised my arms to pull it down, and he said "NO! Put your arms down and out!" I did as he said and with an extra push I was in!! I gave him a very loud "YOU JUST MADE OUR TRIP! I LOVE YOU!" I was more excited I fit into one of those seats than I was about the ride. LOL
My daughter absolutely LOVED it. She was so excited we got to share the experience. I'm glad I ride with her but I would never ride it again. When we got off I could barely walk and almost barfed. OMG the motion sickness was BAD!! I had a TM walk us behind a "staff only" fenced area because I was scared I wouldn't make it to the bathrooms.

LOL
The only real advice I have is don't wear jeans. Wear bottoms that have some give. I wore a pair of black cotton capris. I really don't think I would have fit if I would have had jeans on.
I hope this helps. I took a picture of the test seats. Let me know if you would like to see it.