These straps should be about 0.025 milliohms if they are truely copper and maintain good interconnection between laminations at the bolted down terminal ends.. Should probably use a stainless steel washer on top to ensure terminal area compression across full width of laminate layers.
At 300 amps that would be about 7.5 mV of voltage drop per strap.
Like China grade 'A' cells, you likely cannot trust they are pure copper. Brass is 95% the weight of copper so with dimensional measurement accuracy it hard to tell the difference by weight measurement.
Brass has 4.8 times the resistance of copper however.
Drilling holes in the laminate structure may not be so easy without possibility of snagging and twisting up individual layers. Would likely have to put external outside binding aluminum plates on ends and clamp it tight to keep drill bit from snagging on individual layers.