Drag Racing Tech - LT1 first start issues

385 lt1 with 24x conversion. Battery is trunk mounted wire from battery to starter starter to block on fender alt straight to battery everything is grounded good. The problem were having is the starter doesn't turn the motor over fast enough. No spark plugs in it and its a slow crank. We have 12.8v at the battery and when its cranking it drops to 10.3v. If we put the plugs in there's no way it'd turn over. The wire is plenty big came in the Burkhart relocate kit. What's could this issue be? First starter was brand new factory but it was broken all we could get was a factory reman trying to get it running before leaving for school again. thanks for any help

What do you mean by all grounds are good? Here's how Spaghetti Menders told me to wire my car an I haven't had any issues. I ran the power and ground from the rear to studs that sit on the firewall. Then from the stud on the outside I ran a wire to the starter. From the ground stud I went to the passenger side head then made another wire that jumps from the passenger side head to the drivers side head. This way everything is connected directly to the battery and nothing has to travel through the chassis in order to find a ground.