How much does it cost to rebuild a Slant 6

Why do you suspect it needs a rebuild? All oil leaks can be fixed easily and cheaply w/ engine in the car. Always do a compression check first. Before buying a gage ($15 @ HF), turn the engine over by hand by tugging on the fan belt. If you feel each cylinder (2 revs) and hear each one hiss down, the engine is perfect so don't make problems for yourself. Another poor-boy test is to remove the oil fill and PCV valve and see how much blow-by gas comes out the valve cover at idle. If it isn't puffing, the engine is probably fine.

You wouldn't gain much power or mileage simply from a rebuild, unless the engine was totally shot. Optimizing fuel and spark will usually give much more bang. Somebody smart has been under the hood since it has a modern Sanden AC compressor.