Fh680D bent push rod(s)

While mowing my lawn 3 weeks ago I heard a weird (pht pht) like running over a plastic bag - if you know that sound, then I had 1/2 power on the mower. I searched the internet and it sounded like it could be coil wires so I replaced both and it still had 1/2 power. I guess I should have checked the spark first. A friend came over removed the valve cover and expossed the 2 bent push rods. also the exhaust valve liner was pushed up and the oil seal was destroyed. I ordered a new head and parts from Jacks and installed them yesterday. Started the mower and did a test cut for about 1 minute ran well - I was pleased with my work-first time at doing motor work, then as I was putting the mower away there was a small pop like a very small backfire just before I shut it down. I thought that did not sound too good so I started it up again and -you guessed it only 1/2 power. When i opened up the valve cover my intake push rod was bent and the exhaust rod was out from under the rocker arm. I am not sure what caused this other than maybe I did not set the valve gap correctly. So my question is- When I set the gap at TDC at 0.006" do I set the intake at the same time as the exhaust so they are both the same when the crankshaft is at the same place or do I rotate the engine and set the exhaust when the intake is open.? I assume there is a time when both are closed and a time when only one is open and the other is closed. Is TDC a time when they both should be closed? Also I set the gap at 0.004" but I found that the gap should be 0.006"-would that make a difference?  

I purchased new push rods locally and paid a lot more than on Jacks. I installed the new push rods and changed the rocker arms as well - the previous time I did not put the new rocker arms in, I was very careful about setting the gap on the valves and found a kawasaki site that said the gap should be 0.006" so I used that gap and checked and double checked and triple checked the gap. The mower started up and ran flawlessly all afternoon and I cut some pretty long grass - this is only my second cut this season due too a lot of rain in March and April which made the ground too wet to drive on and then on my second mow I developed this problem and that was 3 weeks ago so three weeks of growth plus one area had not been cut yet at all so it is over three feet tall. The mower cut it all but I will have to finish the rest when it dries up as we are expecting more showers tomorrow. I am using John Deere 737 mower that is 10 years old, I have had to replace the electric clutch twice and the front and rear oil seals on the crankshaft and now this cylinder head and related parts but it is a really solid mower.