If I HAD to swap, I'd go with the 4.9, but the better choice for power is a 5.8, and it's a pretty easy swap. I doubt I'd waste money or time swapping in another 5.0 unless I had one, as you do.
I have a 4.9 in my 84 and it works very well for me, gets 15 mpg on the highway/12 city/offroad...