DanEC Dan I have been running and racing the cheap Fel-pro 1447 for 15 years. We have the same header setup. Here's waht I have done,
Remove the header pipes one bank at a time. get a flat edge piece of metal and check that the head is flat from front to back exhaust port. You can also look at the flanges on each header pipe and see if they are flat too. If not a little light grinding to true them up.
I have tryed and run headers on different motors and cars from many years
Choice 1 I double seal the gaskets with Hi-Tac and let dry. Install the pipes with a coat of antiseize on all 4 pipes. Want a good 1" wide coating. Them install the mufflers and mount them in the rear. tighten the bolts for the header flanges 5 pound at a time until 35 pounds is met. Do this for all 4 pipes. Repeat on other side. Run car for a couple of minutes, outside and let cool off. Retorque bolts again to 35. That should do it. Easy road test a couple of times to help the antiseize seal the pipes.
Choice 2 Reused the copper gaskets with Hi-temp RTV. It's an orange color or black. This is for exhaust ONLY. Do the same thing as above. Make sure you add a little antiseize to the bolts that go into the heads.
Choice 3 Old school is soaking the 1247 in water for 10-15 minutes and then installing them on the motor and drive and soon as possible. retorque them as soon as possible.
Your lifters, I don't know which ones are in your motor but any antipump up lifter don't adjust well. I have run Crane flats and rollers in my motors. Even tryed the adjust at idle with motor running and they don't. I do run a higher
oil pressure and 15W-40
oil. IMO the best way to setup the rockers is without any
oil in the lifters. Anti pumps don't bleed down well. I know this is a pain but would tell you to remove all the lifters and remove the oil from the housings. reassembly them. Some guys like the simple wire clip that comes with them I like to change the clip to a retain clip that you need pin pliers to install. This is what the higher rpm lifters use. Reinstall the lifters and do the basic setup of lash. I like contact and 1/4 turn on adjuster, for street motor. For my motors and racing with hydros I go .015" plunger depression. This helps when turning over 5,700 rpms and the lifters pumpup. The valve will still close and the power drop off is less. My max rpm lim it is still 6,200 rpms.
Last note and question, does the heads have oil limiters in them? You maynot be getting enough oil. The shafts could be upside down and not oiling the rockers. Also IMO a .80" oil limiter should be used for the best overall oiling of valves and springs. When you remove the rocker covers check for any wear of blueing of the pushrods or adjusters on the rockers. This is a sign of no enough oil getting to the correct points. I reheard the video again and this motor with hydro lifters is way too noisy, even with exhaust leaks. call if you want to talk about this. Rick L. 7322543536 in the evening