I think a bypass is way worse than a transplant!
I had terrible back pain. I had my neighbor look at me since he is a chiro. I'm assuming you had a lot of IVs hooked up to you. I had so many lines in me for such a long period of time that my posture was terrible. My shoulders were so rounded and hunched forward that I had to do a lot of mobility work to reset my shoulders. If you want, I can link all the exercises I did that helped me. Foam rolling also helped for that.
I went back to the gym, lifting, 8 weeks after surgery. Keep in mind, I was 23, so healing was probably quick because I was younger. I got the clearance from the doctors. But even then, I went super light on everything. Bench, I would only use the bar. Pec deck or flies, only partial range of motion, the top part. Everything else was pretty easy to get back into, but chest scared me, always. I thought for the longest time my chest would split open.
One thing you can do to see how your chest is recovering is by jogging. I tried it five weeks after surgery and it was terrible pain. I thought my chest was going to split open, everything wasn't fused yet. If you cannot jog without pain, you should not be lifting, in my opinion. How often do you get chest x-rays?
Something else to help with fusing, get a pillow, if they did not give you one, and brace it every time you sneeze, poop, or need to bear down for any reason. It is similar to a C-Section in that manner. I brought my little pillow everywhere with me. Even coughing or sneezing can be painful and slow the recovery process.