Comment by johnklos

There are ways to do this transparently if you control the machine doing NAT for the classic computer. Doing things that way used to be more common before https became the default for most sites, because nowadays you'd have to install TLS certificates on each client to not constantly get errors. However, if you're also proxying https to http, then no worries!

I should update my transparent proxy how-to to be used with MacProxy Plus...