Hey Jeffrey! Never tested like that, so I have zero advice. Just wanted to wish you luck!!

I feel  if you've welded before you should be able to set the gun. Listen to the sound when it sounds like frying bacon you're close. This is a flat test so it should be fairly easy. Dont worry I bet it'll be fine. Get some scrap to set the machine and practice for 15-20 min. If this is big stuff they my want the test in a spray mode??

http://content.lincolnelectric.com/pdfs/products/literature/c410.pdf
pg 13
http://content.lincolnelectric.com/pdfs/products/literature/c4200.pdf
pg 69

for me i would ask for a set up piece of scrap and dont look at the numbers on the machine
start around 30 volts and adjust the wire accordingly. if you feel she's a little cold go up a little  remember this is most likely going to be full spray transfer unless they use pure CO2. and if it is pure CO2 you'll have so much penetration you will not have to worry about it at all
depending  on the plate configuration (run off tabs) i would start at one end weld approximately half way and start at the other end and weld into the middle making sure that my next pass does not have a stop in the exact same spot. it will give you the best chance of getting your weld to the ends without any great big craters and it will give them an idea of your tie in capabilities. one thing is to make sure you dont try and make your cap from to deep in the groove take it to almost flush before you cap. this is actually a very fast test maybe 4 to 6 passes depending if they let you weave at all. just pour the sauce to it and she'll be just fine, after all its just welding and you've already proven you can do that so no worries, that plate is gonna be  some hot when your done.
darren

Ask if they have any 1"x1" angle iron that you can cut about an inch off of for run off tabs (put in on the end of your test plate so it somewhat matches your bevel angle, it won't be exact).  This will help you get the 1" thick test plat full all the way across and look really nice in the actaul test plate area.  A lot of guys will be under filled on the ends of the plate because there is not enough build up due to the molten metal wanting to just run off on to your starting tabs.  Do not half ass the fitup for your root pass. Make sure you have a consistent gap all the way down and a consistent root face. Look to see that the bottom edge of your test plate is tieing into the backing strip.  For spray transfer, you want to use a push gun angle for all you passes.  Build your stringers up to flush out the plate and then run really fast stringers for the cap.  This will help get rid of the undercut you will likely see on your last stringer if you go to slow and try to make up for not having your coupon flushed out. 
     You are welding this coupon in the horizantal position and code lets you kick your plate back at an angle, but you might want to ask if you can first(DON'T say "well code says you can" because then they'll be gunning for ya most likely thinking your a know it all "maybe".  Run that root in hot with a good amount of wire to compinsate, probably 30v and somewhere around 550IPM ought to be close.  If it's real noisey then back the wire off because it should sound nice and smooth with a nice ssssssshhhhhhhhh.  After the root, tame her down a bit.  When you get the coupon flushed out, ask or go for a pee to let let your plate cool down for a minute before you cap her for best results.  Hope some of this helps ya out!! Good luck!!

Mess with them a little and ask for a welding procedure. 

Always push with GMAW for the most part.  L56 wire is nothing but a brand name the AWS # is still E70S-X. 

.052 will require a lower wire feed speed than .045 or .035. 

In the flat position you should be able to run 27 to 32 volts and WFS around 300 - 500

It will take more passes than 3/8" plate but who cares you just weld till it's full.  Keep your cap with no more than 1/8" reinforcement.

Now go to the lincoln web site and pull the specs on that wire, they will give you a general range as to WFS and V