Steam Pipe Pitch

We have finally gotten around to redoing our old enclosed entry porch with new windows, entry door, 6 in. of insulation in the walls and 20 in. in the ceiling. This is a 6 ft. X 14 ft. space with a 9 ft. ceiling. The bolier was slightly oversized when installed with heating this space in the future in mind and doing so will actually match the boiler capacity almost exactly. The radiator we plan to install is an ARCO thin tube with an EDR of 28.8 sq.ft. witch matches the heat loss of this area almost exactly. Since the heat loss of the rest of the house matches the other radiators almost exactly too this should be ok. The radiator was originally piped with 1 1/2in. pipe and there is a capped elbow on the 2in. main where the original radiator for the dining room (witch was removed years ago)was also piped with 1 1/2 in. pipe . Since the radiator we plan to install is directly on the opposite side of the wall from where the dining room radiator was. I am planning on using this fitting on the main to connect the new radiator. The new pipe run will be 7 ft. in length. My question is this, there is a clearance issue with a large sill beam (old House). The maximum pitch I can get is 1 1/2 in. total over the length. after pouring through The Lost Art. I'm still not sure if this is enough. So I thought I'd ask you. Thanks for any advice in advance.