1970 Dart with factory dash tach and I'm installing an MSD system (6al, and pro billet distributor).

Can I just hook it to the tach output on the MSD 6AL box?

I have the same setup. I tried hooking my stock tack to the msd box and it still didn't work. I heard you have to run a tach adapter. I ended up using a Autometer tach.

With factory tach a MSD adaptor is normally required. If you search Summit or other vendors you will find several adaptors listed... pick based on description and wire per instructions. I know on my '70 Rally tach I tried just hooking up the wires and no reading at all. With the adapter recommended it worked... sort of. Not real accurate but it worked. In my case the recommended adapter was the less expensive version.

normally the trigger wire (usually green but maybe yellow on factory) that would go to the negative side of the coil goes to the tach output on 6AL. But being factory may need this tack adapter
http://chucker54.stores.yahoo.net/casehousings.html

How are you triggering the MSD MSD dizzy points mopar dizzy??
Says if you are triggering through the white wire since your using MSD dizzy I'd thing just wire to tach output??
bunch of stuff hear
http://www.msdignition.com/page.aspx?id=3300

adapter wireing
http://www.msdignition.com/instructions/Products/Accessories/8910.pdf?terms=8910

http://www.summitracing.com/int/parts/msd-8910

Since my tach was fairly slow responding, I just bought a reproduction one and sold the original for what the reproduction one cost. You could also have the electronics in the stock one replaced.

I hooked up the adapter on my MSD 6AL on my 70 challenger. The tach works now but it revs slow and it is off a bit. I did find a thread about using a capacitor in-line to adjust the tach adapter a bit.
I have not tried the capacitor yet.
If anyone has put the extra capacitor in line that has some experience with it, please tell us how it worked.
Thanks

I tried using the MSD adapter with a factory tach and was unsuccessful. Had my factory E-body tach rebuilt and upgraded. If you do have your factory tach gone-through, you may need to specify that you will be using a MSD type ignition system otherwise they may not set it up for this.