Don't do it...thats the short answer.
Long answer- They are heavy and run hot! They are big and harder to fit in small areas (around suspension, exhaust, brake booster...). I have been down this road and can say the only upside I have enjoyed is the sound! Yes the power is good, but a decently built small block will keep up and not have the downsides.
That being said, I did enjoy running my last build (440 powered Ramcharger) up the dunes...the nearly 500 hp was fun and turned heads...but it sucked in rock crawling...heavy and I could feel it!
I just don't see the upsides outweighing the downsides. I highly doubt I will ever do another big block build for off roading...now my 68 Plymouth, hmmm the 440 will stay in there.