As someone who owned an FE and converted it from 4-speed to PowerGlide, let me be the first to say: "Don't do it".
PowerGlides are more or less bulletproof, but they cost a fortune to get rebuilt, and they are hard on fuel and hard on brakes: with only two gears they're thirsty, and with virtually no kick-down you get no engine-braking benefit.
In any case, you'll need to put a bigger tunnel in - no auto will fit in the standard grey crashbox tunnel. Whether you fit a larger brake pedal is up to you, but you'll definitely need to alter your tailshaft as well.
I subsequently converted my FE to Trimatic, and to be honest I never looked back. Whilst the PRNDL of an EK auto column won't be exactly right, it's probably close enough. The selectors of a PowerGlide are on the wrong side, so connecting it up to a column shift is difficult - the connecting rods from a HR/HK/HT won't suit the shape of the FE/FC firewall/tunnel. (Mine was floor-shift, so I didn't have that particular problem).
If you're thinking of an auto conversion, get your car/bike nut mate to do it for you, or just go Trimatic and we shall never speak of this again
Oh, and Grey + LPG + PowerGlide? I think you'd be sorry.
cheers
RET