From what I remember, .420 lift at the valve is the most you can run without cutting the guides.
That's an overly simplistic statement. There are factors that using that rule could mess you up. The amount the retainer hangs down toward the guide isn't a fixed distance in all occasions. A set of keepers that moved the retainers down .050" for example, would cost you .050" of available room. A non stock retainer that's thicker, a thicker than normal seal, new seats that move the valve down into the combustion chamber, etc...
Accurate rule of thumb: measure, measure, measure.... then measure again. Then cut if you have to. Lowering guides is really no big deal. Hell, you can buy a drill motor powered cutter than will top the guides in much less time than it takes to post about it.
When it comes to valvetrains, there's always a good change you can get away with SWAGing it... but there's also a VERY good chance that SWAG won't be right. With the amount of damage a shoddy valvetrain can do to our cars, there's really no reason to just start throwing parts on it.
As for the muscle car lope... what you need is a cam with lobes that are too fat, and lots of overlap. That way the vacuum signal at idle is so shitty, the car has that choppy lopey idle that triggers our pituitary gland and makes us dream of big HP.... The problem is overlap blows boost and unburned gasoline right out the exhaust pipe. Big fat lobes kill off dynamic compression and has to either have high compression pistons (or make up for it with boost...that goes right out the tailpipe). Basically the car becomes a dog, with crappy gas mileage, crappy bottom end, long spool up time, and makes peak HP at 10,000 on an engine that can't turn more than 6000ish... oh, and the vavletrain will most likely be unstable and have a limited lifespan.
Even fire V6's don't naturally want to lope like a V8 anyways, so it's that much harder too.
I will admit my car does have a bit of a lope to it.. Not a 'steet freaks' lope, but it doesn't sound like a 6. It does have too much cam in it for what I have, but it was a free roller so I ran it
(plus I cut my own guides down to match the .500+ lifts and increased the compession to 9:1). If I had a smaller roller it would already be in the engine.