Jim, fellow 24v cossie owner here as you well know, the ONLY things really keeping me with this engine are:

1. 4x4
2. Not liking turbos

Cost and economy, fuck that, I want a fast n/a 4wd (car!) as my daily drive. And this is still fun!

As you now are happy with RWD, just drop the biggest fookin V8 you can in it, but remember that you are gonna need to shell out on brakes and suspension for the poor old sierra!!