Engine vibration

Hello everyone,i am new here and would like some help.
i have 1999 4.3 vortec block and 1995 heads and intake.
the block was originaly auto, i wanted manual so i installe 350 flywheel which was suggested by kelly's clutch service here in Atlnta.
when installed the engine was shaking pretty bad,i went for advice and they gave me what they called late model s10 manual flywheel which was much heavier than the first one.
this one was better but it's still shaking.
if you rev the car in neutral you can tell or feel the shaking is coming from the flywheel and bellhousing are,the drivetrain is fine and ballanced fine.
my question is,can vacuum leak cause this?or what exactly flywheel do i need in order to fix this problem?.
by the way engine is installed on mzda rx7 second generation.
the engine is clear from the wall to cause this.
i am not a mechanic but made it to this point and not sure where to go from here.
can earlier model flywheel work on late model engine also or it has to be late model flywheel since the block is 1999?
please help.
dini.