6BT Cummins Swap

[Patience]

Hello everyone --

Pretty new to the site. I've got a 59 Apache 38 Dually that I'm planning on putting a 5.9L 12v in... I've seen a couple on here... most with a monster hole in the firewall and some without any hole in the firewall.

I'd like to keep my firewall cutting to a minimum... What causes some to have to cut the big hole and others to make it all fit?

Thanks for the help! Have really enjoyed looking at a bunch of builds!