Beretta.net

I hope I don't get flame for this, but will the automatic transmission that comes with the Beretta GT v6 work with the 3400 SFI motor?

Yes it will

Grab a higher stall convereter too while your at it, get one high enough and you can probably run some good E.T.s  

It will bolt right up. You'd have the advantage over me becasue your's is  3 speed, but it actually has more agressive gearing than my 4 speed auto.

You might have problems mountign the 3400 though, I'm not sure if all L bodies can mount them up or if only the later years... It should be easier than in a J body though. Might have to weld in a different mount or something.. Not sure. Can't remember anymore.

Actually, unless it's less trouble to drop the 3400 in, you could do this too:

3400 top end swap. Heads, intake manifolds etc. Keep your 3.1 gen2 block. You could get a cam for the gen2 block and with the 3400 upper stuff make very good power.

I'm not sure if they offer a cam yet for the 3x00 series yet...

Ok, last question. I have a friend that has a 3800 with 20 km's on the motor. The bottom end needs to be done on the motor, but to do a swap for this motor, what all do I need?
Will I need a different computer, and will my auto 3 speed tranny work with this motor?

3800 is a totally different animal. You'll need computer, all the wiring, and weld new motor mounts, etc etc etc. It's not an easy swap.

Stick to 3.1/3100/3400 motors for easiest swap.