I know it might sound like a bit of a headache....
But...... IF you've ruled out timing.....pop the valve covers off..
watch it idle...
watch for all of the rocker arms to be moving the same amount as each other..
I dont want to be mr doom and gloom.... but as I wrote before it is possible that your cam is just worn out...
I'm writing this again, because it is important to rule this out. Especially if you cant seem to find another reason for the power issue. Sorry for the already-dead horse I'm beating again!
My 360 ran fine in december of 2010.... tranny went out... I ran the heck out of it that night...It took me a year to get off my butt and put some time into my Jeep... When I restarted it in december 2011, there was a ticking and total loss of power when I started to put my foot in it. I put a new ignition system in it because i thought it was a timing problem. I was considering putting a new carb on it as well because I thought it might be a carb issue.
When i finally decided to pop the valve covers, I noticed there were 3 rockers that were noticeably moving far less than the other 13. After determining that it wasnt a bent pushrod, or collapsed lifter, I pulled out the camshaft, finding 5 cam lobes that were nearly completely round.
Just prior to discovering this, I had bought another 360 from a guy who had already disassembled it. My first thought was I could potentially use the camshaft from that engine to swap temporarily into mine until I could rebuild the newish 360. When I dug it out of the pile of parts on the shelf, guess what??? It had 4 lobes nearly worn out as well.
Since the government (its ok, we're here to help) took additives out of motor oil, worn out camshafts with flat tappet lifters are becoming more and more common. Luckily I had a
AMC 304 sitting on a stand with about 30K on its cam... I took it out and swapped it into my 360 and its all good now...
But for some reason your mechanic said it could be a broken valve... I wonder if he felt it wasnt hitting on all cylinders and drew that quick conclusion.
Since you have basically ruled out timing, take the time to pop the covers off and watch the rockers move... the only thing you will have wasted is a set of valve cover gaskets...
As I wrote before, this may NOT be what is going on, but it is becoming more and more common to see this type of problem...
good luck....keep posting what you find.
BTW: anyone reading this should begin using additives in their oil to prevent this type of issue.
brian