There is no simple solution that springs to mind, but the simplest I can think of right now is to use a Cat5 Ethernet cable to join them. 300ft is the quoted maximum length for Cat5, and it will actually work beyond this range if it is good quality cable and installed carefully.
Then find and install an ISA network card see click here for details.
Finally, setup ICS on the newer PC, and tell the Win 3.11 PC to use it as a gateway.
P.S - the procedure for NIC installation is very different for Win 3.1 - see click here in case you made a typo.