Mercruiser 7.4 ignition help

I got a Mercruiser 7.4 that i have been servicing for 6 years, and they hydro locked the engine and busted a rod and slung it through the block. I installed a new engine and went to fire it up for the first time and it wasnt getting no fire. i put a new cap, rotor, and coil on with the new engine. It has 12 volts on purple wire going to coil when its not hooked up, when i go to hook it up it arcs bad like its shorting out and drops to 8 volts. I called the other merc tech i use to work with and me and him both said it sounds like a bad coil. I bypassed the safety switch, tried another new coil but nothing. i hooked a spark checker straight to the coil and grounded it and it will spark for a split second when you first start to turn it over then lose it. I need some help i gotta get this boat turned and out of the shop.

Check your coil grounds.

Thats the first thing i done, ive checked all my grounds and they are good

see attached

disconect the gray tach wire from the coil and try it tack can be shorted even if it works. if it still dont start put tach wire back on turn engine over if tack moves dist. pick up is good let us know what happens