I've never tried putting a 68LC040 into a Mac that came without one, but since SANE checks for a physical FPU before dispatching FPU instrutions, you shouldn't, on paper, have any trouble with a swap like this.

If you had a spare drive, I'd try swapping that into the Quadra and doing a clean install. (the spare drive would ensure you don't nuke your existing data just to find out that it won't help. )