engine balance problem

[Fyrpilot]

My son bought a 1987 T-10 and brought it home. It has a 2.8L V6 (RPO LL2) (VIN code R) TBI engine. It vibrates like hell at idle with the transmission in park. It has good compression and spark on all six scyinders, so I'm certain that it has the wrong flexplate. The engine number is: A930111AAZ.
I'm pretty positive that the engine was swapped out by the previous owner and he was unaware of the internal/external balance difference. It has a 700R4 trans. Obviously I would like to acquire the correct flex plate before disassembly.
The vibration is so bad that it has broken several brackets. It's a solid truck, but I can't afford to buy a 3.4HT for it right now.
Can anyone tell me whether or not this is an internally balanced engine? Does anyone know the part number for the correct flexplate?
I would deeply appreciate the help.

thanks,
Fyrpilot

[geoffinbc]

It is very possible that there was a mixup. Plus 87 what the transition year so the engine could be either internal or external balance.

To ID the engine you need to look at the rear main seal or the crank itself.
If you remove the transmission, which is something you will need to do anyway. Look at th rear main seal. If it is a one piece seal the engine is internal balance. A 2 piece seal means it is external balance. If you want you can also drop the oil pan. If you see a reluctor wheel (large metal disc) right in the center of the crank then you have an internal balance engine. No wheel means external balance.

An internal balance motor requires a neutral balance flywheel/flexplate. A neutral balance flexplate/flywheel will only have small weights or balance holes around its perimeter.

An external balance motor needs an offset balanced flywheel/flexplate. In this case there will be a large mass cast into the back of the flywheel or a large weight welded to the back of the flexplate.

So you need to make sure you have the right match. Or in this case since you obviously have a balance problem you need to switch out the flywheel/flexplate for the opposite one.