Rpm related vibration 5.9 magnum

I want to apologize in advance, this post is going to be long but in order to solve this there is a lot of information that needs to be shared to get an idea of what’s going on here.
I’ve got a real brain stumper on my hands here and I’m dumb founded and have no idea what’s going on, I need some serious help determing the cause of an rpm related vibration that I have.
Engine is a 300hp crate 360 magnum, mileage is just over 30k miles on engine, it was purchased brand new in 2003 by my father.
This vibration comes on at about 2300-2500 rpm.
Doesn’t matter what gear I’m in the vibration will be the same at 2500 rpm in every gear regaurdless of speed.
So I know it’s not drivetrain related, which for the longest time I thought it was.
This engine is in my truck, it’s a 91 w150 short bed 4x4.
I’m running a 2 pin non lock up A-518 auto, I have an LA style flex plate with a Magnum weighted torque converter.
I’ve verified the converter with other magnum weighted converters and they look identical.
Transmission was re-built a few years back, below 2500 rpm the truck runs nice and smooth, no vibrations what so ever.
I have the La style timing cover and v-belt set up which is what came on this engine as a crate.
I have brand new ps pump and ps pump brackets, brand new alternator and alternator brackets.
I’m running a double grooved crankshaft pulley and a single groove water pump pulley.
Belts are brand new and straight, everything is tight.
Fan is a brand new 6 blade flex fan bolted directly to water pump, no clutch fan.
Harmonic balancer is the balancer that came with the engine from Mopar as a crate, it’s never been swapped.
At first I thought this was an imbalance issue, but now I’m not so sure and I’m leaning towards some kind of issue with ether the valves or the valve springs in the heads.
Here is why, I ran the truck in park up to 2500 rpm and the vibration wasn’t as bad as I expected it to be if it were an imbalance problem.
There is a rough spot at 2500 and when I open the door and listen to the exhaust closely I can hear it popping and missing like it’s breaking up and not hitting evenly on all 8 cylinders.
I was fighting a miss fire issue a month or so ago and I thought I had narrowed it down to a faulty crane cams hi-6 cd ignition box.
But now I’m not so sure.
I did buy a brand new MSD Digital 6 box and wired that inn with all brand new wiring and I bought a brand new crane cams lx91 coil to go with the box.
I ran the trigger wires and the coil wires on opposite sides of the truck so they are as far away from each other as possible to prevent false triggering.
I’m running a brand new firecore 50 electronic vacuum advance distributor.
The mechanical advance is set at 16 degrees, initial timing is set at 18 degrees and total timing is set at 34 degrees all in at 2500 rpm with the 2 blue medium tension firecore advance springs.
I’m running brand new Ngk v-power bkr5e plugs gapped at .040.
Plug wires are brand new Taylor Thundervolt 8.2mm wires and all 8 wires ohm out at 40 ohms of resistance per foot.
I’ve verified my timing with 2 different lights, both are within 1/2 a degree of each other.
I have verified the timing mark on the balancer using a piston stop, the timing mark is accurate.
I’ve done a warm compression test on all 8 cylinders with the carb wide open.
I get 155psi on all 8 cylinders.
This vibration feels like it’s got a rythem to it, I can feel it in the steering wheel, in the floor boards and in the seat, and it seems to have a steady rythem to it like it rotates around with the engine, going 55 mph in 3rd gear down the highway puts me at about 2400-2500 rpm with 4.56 gears and 33 inch tires, while going 55 in third the vibration just goes womp, womp, womp.
Then when I shift the trans into overdrive and the rpm drops it ether clears up substantially or it completely goes away.
When I first got this firecore distributor is was suggested to me to slow down the advance rate with heavier springs.
So I swapped in one light silver spring and one heavy black spring which slowed the timing being all in to about 3400 rpm.
This didn’t help the issue and took away a lot of power, so after I wired the new Msd box inn I decided to switch back to the medium blue advance springs.
When I did that the vibration seemed to change, it got slightly better??? I don’t understand how or why that made it better.
I’ve also discovered that if I dis-connect the vacuum advance it will lessen the intensity of the vibration going down the highway but it doesnt eliminate it all together.
I am seriously at a complete loss as to what’s going on with this engine.
All I know is it’s an rpm dependant vibration, not speed dependent, and I don’t know if it’s imbalance or an engine miss, but I’m leaning towards engine miss because like I said I can hear the engine breaking up a little bit while in park at the same rpm and I have verified my converter and flex plate are right.
Since all the ignition parts are brand new and timing has been verified, I’m
Leaning towards something going on with a valve spring or a valve.
I just need to know how to confirm this.