For a low heating shoulder season month that uses 50 therms that's 50 x $0.50 = $25 for the gas, $85 for the delivery, $110 total for 50 therms, or ($110/50=) $2.20/therm.
Impressive analysis, as usual, Dana.
I just want to point out that because the OP already has gas, and is using it for other purposes, the financial analysis of oil vs gas absolutely should not include the fixed ($85) service fee. This fee is going to be paid regardless of whether the OP keeps oil or converts to gas. The analysis should only include the incremental additional cost of the gas to be used for heating the house. This means the cost/therm used for the analysis should be lower than the figures used, and will be much cheaper in the low season, so the actual savings should be much greater than $350.