The following is from another post I made on this:
There are several different dampers. Both the diameter, thickness and the placement of the line on the outer ring of the damper can differ. So you need to know what you have, to know what tab to use.
The three most common locations for the timing mark on the damper:
The exact years of the type of damper timing marks overlap one another, depending on the exact application.
The pre-'69 damper has the TDC line on the outer ring at the 2:30 o’clock position- or 2º before- (i.e. to the LEFT of the keyway, looking at the front of the engine) the keyway centerline. The keyway is seen in the ID of the damper.
The damper used from '69 to about ‘84 has the TDC line at the 2 o’clock position- or 10º before the keyway. You'll find that aftermarket dampers are the 10º type, as are the bolt-on tabs sold in the aftermarket, unless they're adjustable.
A third timing mark was used from about 1978 to about 1995. It is at the 12 o’clock position- or 40º before the keyway. This damper uses a timing cover that has the tab welded on at about the 12 o'clock position. Professional Products lists the years for this type damper line as being ‘84-’95 and is a 6-3/4” diameter damper.
If you're unsure, or just want to check the accuracy of the tab/mark for TDC, DETERMINE TDC and MAKE A TIMING TAPE will help you to see for yourself.
The 400 should have the 10º TDC mark position from the keyway. There is just the one 8" OEM balancer for the 400, AFAIK
You may have a pre-'69 damper on the engine. Problem w/that is this damper was before the 400 came out in '70- so it would be a neutral balanced damper, not scalloped like shown above for an external balanced 400 SBC engine.
If the tab shows 4° ATDC- and the damper is correct for a 400 SBC- the easiest solution is to just remark the damper w/a correct line for TDC and ignore the original line.