If you want to use OS 3.9 at any sort of decent speed you will need a 68030 accelerator at the very least.
With your 6MB Fastram and 1MB chipram you are at the bare minimum ram-wise for it to run.
I would look for a 68030 board, even a Commodore A2630 board should be enough but if you want to push things a little further a GVP 030 or 040 Combo are even better.
The GVP Combo accelerators (030 or 040) will allow you up to 32MB of 32Bit Fastram. The 32bit fastram will greatly improve speed for you.
A warning though, the GVP accelerators use their own propriety RAM modules and these are pretty hard to find and very expensive.