You don't need to change your gas block for a drop in if you get a carbine length rail, are you set on the longer rail? If you are, then install your low pro gas block and then get a free float longer handguard and return the drop in. There are reasonably priced free float rails. The drop in needs support in the front to attach to the barrel so you can't just slap it on top of the low profile block, it needs a way to support the front and a low profile gas block does not do that. If you are going to replace the gas block to a low profile then just get a free float at that point. The free float only attaches at the receiver end and never touches the barrel at the front, thus why it "floats" the barrel.
Yes, you will have to remove the delta ring and possibly the barrel nut (depending on what rail you get) to free float the barrel, but that's the best way to do it and if you have the low profile block, you might as well. It's really not hard. The hardest part will be getting the old gas block off, the rest is easy.
And yes, sorry tuff love. You should have done your homework first before ordering anything, you need to understand the rifle and how it is put together and works before you start swapping stuff. You ordered a rifle length drop in hand guard and you have a carbine length gas system, it won't work on your OR as it sits.
