Some other thoughts on engine choice:
1) The 460 is a torque monster, while the 4.6l DOHC makes it's power at higher revs. I think the 460 is great for street use - easy to get rolling and lots of torque available any time you need without revving it (which tends to attract unwanted attention from the folks with badges). The only challenge is exercising restraint - it's easy to over-do it.
2) I have a 460 in my West Coast Cobra. It has a 6" longer wheelbase than the Street Beasts' 90" wheelbase and some of that length is in the cockpit, with the rest for the engine. There's plenty of length for the engine, but not a lot of extra width, so I would think a DOHC engine would be a tight fit, and the extra height would require a larger hood bulge and/or scoop.
3) In spite of the extra weight, I think my car handles pretty well, though keep in mind I have not taken it to the track or autocross. It's running 265 tires up front and 295 in the rear, with front and rear sway bars. It also has QA1 coilovers in front and I believe the springs are 465 lbs/in - pretty firm but seem to work quite well.
4) In spite of the extra weight of the 460 on the front wheels and the 265 tires I'm running, I'm giving serious thought to removing the power steering and going full manual.
5) FWIW, if I had a clean sheet of paper I'd be inclined to use a small block, most likely a stroked 351W to about 408 CI. Lighter weight, good torque and HP, and smaller size making it easier to work on. At this point, though, I"m not going to swap out my 460, I'll keep it and perhaps look at aluminum heads 'down the road' to improve the breathing and take some weight off the front end. Just my $0.02.
Decide what you're going to use the car for (e.g. street use only, track only, combination) and go from there. My bottom line is the 460 is a great engine for the way I use it today, but that's not for everyone.
Here's an August 2008 article about a Street Beasts car with a 460:
1966 Street Beasts Corbra Replica - Kit Car Magazine