MSD boxes put about 450 volts to the coil. Hope you didn't burn it out putting it on the coil. Tach terminal on the box only. You shouldn't need a tach adapter.
Like Larry said just attach to the tach terminal on the box I extended the wire on my car because the 6A box is mounted behind the battery on my car away from the exhaust. Factory tach works perfectly fine.
Here's what I used, unfortunately there is no part number on it. MSD had a couple different options and I distinctively remember sending one back. I'll send you this one for $20 plus the ride.
Pm me your address and I'll get it out to you. I'll see what flat rate box it fits in and then send you a total.
Thanks anyway Joe...I talked to MSD and they claim I need MSD 8920 adapter so I ordered one. If it doesn't work i'll get back with you...