Is $6,000 to convert existing oil furnace to gas with a new burner a fair price?

I live in a 3,000 sq ft brick house built in the 1880s in Maine. The house has no insulation and old windows. My current heating system is a 21-year-old Visseman furnace with a hot water reserve, providing baseboard heat with circulator pumps across 3 zones. The price of oil is very high, and the house needs a lot of work. I'm considering converting to natural gas because gas can be piped in at zero cost, and I might qualify for a $1,500 contractor credit for a new gas unit and a $600 federal credit. The house layout is long and narrow with a dog leg, which might require a larger heat pump system if I were to go that route. Currently, I use window units for AC in the bedrooms, which are loud but fine. Insulation and air sealing are definitely on my list of future projects, as there's not enough room between bricks and plaster/lathe for more wall insulation. I'm also adding a small fireplace insert as a backup, safety upgrade, and for entertainment, though it won't heat all zones due to the floor plan. My current boiler is performing well, and most professionals believe it has 10-15 years left, but I'm running numbers to see if replacing now makes sense given the short time horizon for payback.