The fastest quarter-mile ever printed in the era when the Boss 429 was new was 15.29. Much like the Hemi, the Boss big-block was essentially a pig as a production-line engine. Both were detuned race engines, rather than hopped-up station-wagon motors like virtually everything else. Sadly for Ford, in any type of racing where it was pitted against the Hemi, it still got clubbed like a baby seal for the most part. I think a lot of that came from the fact that the Hemi had 4-5 years' worth of development on it by the time Ford introduced it. The current "Ford" Top Fuel engine is a Chrysler Hemi with a couple of bolts moved around, nothing more, just enough to be "Ford specific".
The reason for the 347 is more about weight and size than anything else. A Cleveland is a tight squeeze in a lot of Fords and weighs a ton more than the short-deck Windsor (it's even a bunch heavier than a 351W). If you really want to build an evil little Ford engine, Edelbrock makes a 100% bolt-on Boss-style head & intake for the 302 block. It saves a bunch of effort over the time-honored "Clevor" design, which requires the hard-to-get Track Boss intake. I seriously considered the E-head combo several years ago when I found a '72 Maverick Grabber in a yard near here; my Dad had one when I was a kid. I ultimately passed on it, but the car's still there. The problem with the Boss 347 idea? Well, for starters the real fun doesn't start until about 4,500RPM.
As cool as the late model Hemi is, it's very pricey to acquire and build whereas LSx engines can be had for a song and respond amazingly to even minor modifications. I had a chance to buy a 345 Hemi out of a two-year-old wreck about 7 years ago for $600, but after looking at what it would cost to modify, etc. I ultimately decided to stay with the LA engine. It's a great swap, and I know it's less expensive now, but my pockets still don't run that deep.