Your right I had a brain fart. They are not the same engine family.
Reguardless the 390 does fit in a sn body. You will need a few things and will most likely run into a snag ot two with such a swap but we are here if you have any problems. Since you have a SN your stock brake booster shouldnt be in the way for the driver side valve cover which is a plus. That said, you should only need basic swap equipment to make the swap work. The engine, K member, custom oil pan, swap headers and remaining exhuast, steering rack, engine mounts, trans conversion plate adapter, thin profile radiator and electric fan setup. You may strugle with getting guages to work but we can help you with that when you get that far. I dont know the exact engine height for the 390 but if its close to the 460's 32 in engine height you will need to find a way to make room. There are a few ways to make room, the engine can be lowered in the craddle by way of using engine lowering engine mounts or K member spacers and a low rise intake manifold will help. A few guys have swaped 460 and been able to fit everything under the stock hood with the right parts but you should plan on having to buy a different hood, cowl or otherwize, after you swap incase the stock hood wont work for you. It is also important to see if the swap is leagle for street use in your state before you go through with it.
I would adivse against a swap like this, theres not much performance benifit to be had for the money spent unless you just have to have that "BB" rumble sound.