This post was split from this thread:
http://www.car-seat.org/showthread.php?p=1539272#post1539272
I am assuming you are in Canada since you mention your Marathon having a 30lb weight limit for rear-facing (rf'ing.)
You would need to contact your dealership or go to NAPA auto parts to get the correct piece specific to the vehicle manufacturer. So far as how easy it is to install, I'm not sure. You would need a torque wrench to check that the bolt was tightened to the proper tension. If you already have one of those, and your vehicle happens to have pre-drilled holes, it may be a simple process.
You don't want to buy a tether anchor from toys R Us or another retailer because the threads may or may not fit in to the pre-drilled holes - metric vs. standard sized holes can be a problem sometimes. There is also no standard of testing on after-market tether anchors, so you want to get the one that the vehicle manufacturer has approved for your vehicle.
Using a cargo hook for a tether anchor isn't allowed because they're not tested to be able to bear the same amount of weight in a collision as a tether anchor. (With the exception of vehicles that designate cargo hooks to double as top tether anchors, but your vehicle isn't one of those.)
If it's not possible to get the tether anchor done in time, then rear-facing 1/2 lb over the weight limit would be the safer option provided your daughter has at least 1" of hard shell above her head. I would be sure to install the seat a bit more upright though - anywhere between 30 and 45* measured from upright is acceptable, but I would aim for the 35 to 40* range to minimize any chance that the seat would over-rotate in a collision.
In a comparison of possible risks, rf'ing that 1/2lb past is the least risky if your daughter fits the seat by height still.
Hope that helps.