Slow crank

You mention a new battery, but what is the battery size and cold cranking rating?
Typically when moving the battery to the trunk the battery must be up sized, due to the increased resistance of the longer cables.
Also if the motor is newly rebuilt and on its first starts, it will be tight.
Engines tend to loosen up after they get a few hours on them. If this is the case you may need to jumper in a second battery for the initial starts.