The output from your PC sound card may overload the rig's microphone amp. So you may need a voltage divider (10:1 seems to work) to reduce the sound card output level.
If you have a "line-in" jack for the sound card, the output straight from the rig will probably be OK. If you only a "microphone" sound card input, you may need another voltage divider.
You need some way to key the rig from 'receive' to 'xmit'. VOX works OK, if you have VOX, and set your computer to "No Sounds" using the Windows Control Panel. Otherwise, you'll need a keying interface.
I would put split ferrites on both ends of the connecting wires. A better way to avoid RF feedback is to use two transformers, so there's no direct coupling between the rig and computer.
With AFC enabled on the PSK31 software, even a late-'70's rig may have enough stability to work well.
The alternative is to buy an interface box that includes all the circuitry. I used (and recommend) the Rascal from www.buxcomm.com
which is about as cheap as they come.