I have an 8.1 install of Slack and I installed the CUPS 1.1.14 package from the extras directory that I had from the slackware-current tree. I used the pkgtool program to install it. I also enabled SWAT and setup the printers. I ran the "/etc/rc.d/rc.cups start" to get the daemon going.
I use a remote HP laserjet pinter with a JetDirect. It appears to be working just fine. I have printed the CUPS Test page and printed from the console prompt, KWord, and Abiword.
I also have a local parallel port Epson Photo 700 that appears to be working fine. I printed a doc from KWord with color text and the CUPS test page.
The conversion process I would think would start with the filter files. They are located in /usr/lib/cups/filter. I have 13 files there and they are called as required by cups to process the print job for the specified printer driver.
Also, I do not have any users setup on this machine so all was done above as user root. If you were a normal user then you might try root to see if it is a permission problem.
If you used a package from the second 8.0 CD then that might be the problem. You would then need to download from a mirror the right package for CUPS 1.1.14 that was built for 8.1. I only mention this because it doesn't note what version CUPS you installed.