cnichol, you CAN use 15x1 needles for straight stitching, I'm doing that right now for FMQ on my 319. I *think* you can also do a zigzag/decor stitch with these needles if the width is under a 3. I have not tried that yet, but if I do I'd operate the via handwheel first to see how it goes. I did order an after-market bobbin case with the intent of modifying it, but the machine does not like it very much and gives me some trouble. I'd like to try the modification, but hate to risk my only vintage case.
I have also tried the serger needles -- be careful to get the ones with the flat side. The sewing machine store gave me the correct size but entirely round, which of course didn't work. If we didn't experiment, we'd never learn anything! Enjoy your 319, they are wonderful machines!