440 question

[69dodge383]

I noticed my 440 was running rough when I started it the other day. Did a compression test and 7 cyl were at 185psi and 1 cyl at 155. I had started it a few weeks before to let it run for a few minutes. When I went to rev it, it hesitated a little bit and I figured it was just because the engine was cold and when I rev it again it gave a backfire in the area of the carb and another backfire almost simultaneous that felt like it was in the driver side of the motor in the cylinder. It started running rough and would back then backfire though the exhaust. Thats when I did the compression check and the cylinder that had the backfire is sure enough the one with the 155 psi reading. I am thinking I burned a valve because the timing was off some. Does this sound right? If that is the case...can I repair it in the car without pulling the motor or does it need to come out of the car. If I have to pull it out. I thought of swapping in another 440 I found for a good price. It is a 1967 440 out of a chrysler imperial and ran when it was pulled. So if I have to get the other 440 engine while I am repairing the other 440. Can I swap in a .509 cam in the stock '67 440 and run it as is so I can use the same converter with the 440 I have in the car now?
Any thoughts on this would be appreciated.

[metallicareload99]

Was the "bad" cylinder the last one you tested? How old is the battery and was it fully charged? I usually leave a electronic battery charger hooked up to the battery charging while testing.

I would try and run a leak down test before doing anything else. Maybe pull the valve cover and rockers and inspect all that, especially the lifters on that cylinder