Oh my bad, I was thinking it was a '74z28 ha ha. So it's an LT1 car, still might be worth a try to eliminate those parts.
Might be a dumb question but are you dead sure you got the LT flywheel and not the LS flywheel?
They are only one year apart '97 being LT and '98 being LS.
I have the RAM aluminum flywheel and the LS7 clutch in I my '01. I also opted for the ARP flywheel and pressure plate bolts.
You at least used new bolts I assume?

 

Does the car vibrate on decell the same as it does acceleration?
If the car still vibrates with the car in gear and the clutch pushed it is not trans related.

You can download free software for a smart phone to analyze vibration. Set the phone on the engine with the engine held on a constant RPM. You should see clear vibration with some noise around it, Ignore the noise or low amp vibration, convert the Mhz of the vibrations to cycles per minute. If cycles per min matches Rev per min then the Vibration is coming from Crank/Flywheel/Pistons assemble. If its half the then is valve train or misfire.

I have experienced fly wheels and even transmission coming loose the vibration is 10 time worse as you let the clutch out. At least in the cars I worked on.

 

double check and make sure you got a external balance flywheel. lt1 is internal balance front, external balance rear engine. I ended up with a non balanced flex plate in my auto and had the same issue

 

so I got the steel RAM flywheel from hawksmotorsports, I just checked it is for an LT not an LS. It does look different than the one that came off, the one I took off had a bunch of holes in it and the one that went on didn't. I did use brand new ARP bolts for the flywheel as well as the clutch.

the car vibrates the same no matter on accel or decel. and it does vibrate when the clutch is pushed in and revved. I just have a hard time believing that the car developed a vibration while it was down from the rebuild.

could this be a problem with old gas and bad detonation at higher rpm?

 

did you mark the crank to make sure the 7th hole is lined up in the same place as before?

 

I'm almost betting it's the flywheel, though I would think the Ram flywheel would be set up for external balance, but being aftermarket, it may not be. Only reason I say that is because you say it was running fine prior (minus the trans issue) and this developed after everything was back together. Pulling the trans sucks, and I would exhaust all other ideas first, but I feel that's where you're going to end up.

 

I didn't mark the crank prior to removing the old flywheel. the flywheel has an alignment pin so I didn't think to do this as the flywheel can only go on one way.

 

what part number on the flywheel? maybe try repositioning the pressure plate. did you mess with any of the spark plugs or wiring? a missing cylinder or crossed wires can sound right but cause vibration but also comes with loss of power. doesn't sound like that's an issue with you though.

 

summit shows the 1554 is external balance which would be the right one. not sure if they mark the part or not. might call hawks and see if its not supposed to have balance holes drilled. summit shows the backside with the holes.

 

I didn't notice any holes drilled on the back side of the flywheel before putting it on, but I didn't think about it. the picture on the hawk website also shows them drilled. i have seen people on other sites saying there are 3 different balance weights. i wasn't given an option prior purchasing the flywheel, so i assumed they were all balanced the same. how would i know what balance i got or what balance i would even need if i decided to go with a different flywheel?