Automatic trannies
Chad,
First of all, horsepower is not a determining factor for power handling limitations,
torque is. Stroking a 351 to a 396 will create a torque monster. You're gonna have the torque of a big block.
There are some racers out there who swear by a C-4 that are running in the 10s and quicker, however, they really don't care about longevity. A C-4 has a lot less rotating weight than a FMX or C-6, that generally makes a C-4 car quicker. A FMX or C-6 have the built-in torque handling capacity of a built engine and will last longer. What trade-offs are you willing to make? Let's put it this way... Ford never put C-4s behind big blocks. That should tell you something! The lightest-duty tranny they put behind a 390 was a FMX. Heck, even 351-4V cars had FMXs. So do many 302 cars! Even my 66 Comet with its 289-2V has a cast iron Cruise-o-Matic (aka FMX).
A car driven on the street will be shifting a heck of a lot more often than a race car. You have to take that in mind. Also, there are many tricks that can be made to the internals to take the slop out of the system. Then there are ways to make the clutches react faster, harder, etc. If you have never built up a transmission for all-out race duty, I would strongly suggest leaving that to the professionals.
What I
would recommend to the novice is the installation of a shift kit. Don't get the cheapest kit. I believe B&M has both a Shift Improver Kit and a Trans-Pak. The Trans-Pak gives you many more options, mild to wild. I believe you can even make your existing valve body into manual-only if you so desire. I chose the middle option for my C-6s. That allows automatic functioning in Drive, but full downshift capability at any speed if selected manually. I found that out the hard way...
Before the installation of the transpak, if I were to manually select L while travelling down the freeway, the transmission governor would only let it go down to Second. Afterwards? Well, one day, when coming back from the track, I was messing around with one of my buddies on the freeway. He came charging up next to me so I crammed the shifter down to L like I always had to downshift (my carb did not have a downshift linkage). The transmission obeyed by going right into L --
at 60mph! 
Man, I thought I was gonna blow the engine! After the smoke cleared, and the car stopped bunny-hopping, I realized what had happened. The tranny went into L and the rear tires obeyed the engine rpm, so the rear end was sliding and hopping all over the place trying to get traction again until the engine speed caught up with the tire speed. I'll never make
that mistake again!
OK, Like MM, I would advise against putting in a manual transmission. Too much is involved to do the swap. You'll be much better served with a stout automatic. Use either a FMX or C-6. They won't be all that hard to find. Almost all older pickups and vans with small blocks will have one or the other. Time for a little junkyard scrounging! Then again, if you want any sort of gas mileage out of your car, you might consider the AOD. Be careful though, you don't want an electronically controlled AOD! You can use up to and including a '89, I believe.
Good luck!