The 420 is a new, and very fast bow.
Likely that fixed blades (regardless of selection) will "
plane" or steer your arrows at those speeds.
I know they do with my 380 Matrix, and it is slower in comparison.
As for arrows, you will want the
stiffest you can find.
Black Eagle Zombie Slayers are the choice of most of the dedicated Excalibur shooters for their faster bows.
They are what I use.
With a Luminok (for me) and an heavy front brass insert to balance.
You will want to keep the overall weight (arrow & broadhead combined) right around 400 grains for optimal performance.
There are many mechanical broadheads around that will do the job.
That said, I have been much MORE than satisfied with NAP 125 grain Spitfires.
These fly pretty much exactly the same as 125 grain field points, and I have (commonly) run them to 100 yards to prove just that.
Welcome to your next Obsession!
LOL
BTW: The Excalibur Forum has a bunch of very fine folks on it. Most are willing to help, and there is zero of the drama that is found on other forums:
http://www.excaliburcrossbow.com/phpBB3/
Same handle there. Stop by, post a question or two, and say Howdy.
Cheers,
Nog