Howdy,

My 2 cents worth.

Relocating the battery to the boot is a cool idea.

Things to remember:

Standard lead acid batteries give off Hydrogen gas when being
charged, so they are a fire hazard if placed in a non-vented
confined space.

I = V/R (Current = Volts divided by Resistance)

All cables have resistance, the longer the cable the higher
the resistance.

The bigger the diameter of the conductor in the cable,
the lower the resistance per metre will be.

So I would recommend 50mm squared cable from the battery positive to the starter motor solenoid.

50mm squared cable from the battery negative to the body of the car as close to the battery as possible.
The cars Body will be of lower resistance than running an other cable
because of the cross sectional area of the Metal in the body.

50mm squared cable from the motor block to the Body of the car as short as possible.

For modern engines it is also very important the ECU is earthed well to the body.

The positive feed to the ECU should also be large-ish
(6mm squared) to minimise volt drop to the ECU during engine
cranking.

All connection to the body need to be on bare metal as paint
is an insulator and will increase the joints resistance.
So carefully drill your mounting hole in the body,
then remove the paint around the hole so the cable lug attaches
to bare metal as you want the electrical current to travel through the lug and not the bolt.
then once attached, paint over the lug, bolt and body.

Just so you know,
I've been in the Electrical game both Commercial and Automotive
for the last 35 years and my cars all work well.

Cheers,

Geoff

ps. sealed gel batteries are a good idea and are safer as they do not need venting and will not spill in the event of a roll over . . .
but they will not take as much abuse, eg. long periods of cranking or being left discharged as a standard lead/acid battery will.