How to seal HVAC ducts running through old window to addition?
assumptions from what's in  your photo:

1)  it looks like the right side of the opening is your access, and the ducts are rigidly installed.  

2) the crawlspace looks unheated, as the ducts seem to be insulated, and the fact that you mention radon must mean you have an actively vented (fan) crawlspace to evacuate any radon gas (along with a radon alarm in the living space of the basement adjacent).  I personally don't have any experience with radon tight building, but certain municipalities in Ontario have this as an issue, so its covered in the provincial building code.

if that's correct here's what you can do.

1)  build an airtight, removeable access door and frame assembly to give you access.  you can find this on youtube I am sure, or just copy the way an attic access works.

2)  on the ductwork runs, build a bigger pipe around them so that you have a 1" gap between the heated duct and the slip pipe.  you can wrap a piece of galvanized steel flashing around them, overlapping the ends by approx. 3".  use galvanized steel, not aluminum, to prevent galvanic corrosion.

3)  stuff fiberglass insulation into the gap between the pipes (not too compactly) and seal the ends with fiberglass cord sized to match the gap (1 in gap needs 1" cord.

4)  tape the end of the pipe over with vapour barrier and adhere to the slip and the duct. now you have an airtight insulated joint that can expand and contract and won't cause corrosion on the duct pipe. 

5)  fill the gaps between the slip pipe and everything else with spray foam. fill the gaps from both the cold side and the warm to trap a dead air space between the foam fields. now you have a vapour proof insulation mass that will keep your heat in.  

6)  THIS IS VERY IMPORTANT.  go through all the pipe insulation on the cold side and make sure that all the insulation is in good shape and that the vapour barrier is completely intact.  remember, extra insulation on vent pipes does nothing but save you money and headache when it comes to crawlspace hvac ducting.

now you have a giant section, half access panel, half bulkhead, all of it airtight, insulated and radon-tight.

done