Nice cage, DJ!
I think the cage will accommodate 300 BG.
Be careful crowding BG during your trip - this is where a lot of stress can occur. Do you have a 100G tank [galvanized or plastic]? I think 150 fish in 20G can be done, but isn't ideal. 100G tank is around $60 at local farm supply up here in NE - really good investment.
Due to low temps I think two diffusers in 100G should work for a 1.5 hr drive.
Be sure to gradually acclimate them to your pondwater before caging them. I like replacing 5g bucket every 5 - 10 minutes until I'm 75% pondwater, then I go for it. Better safe than sorry!