that depends on how much of the work you can do yourself. cutting the axle tubes and rewelding them is fairly easy, if you have a welder and a proper tubing cutter. as for the suspension mounts, if you have a plasma cutter and a welder, you can cut off the old suspension mounts, and weld on the new ones and the cost would be buying the leaf spring mounts, which run about $30 iirc. then the only thin you have to farm out is the axle narrowing, and that depends on the shop labor rates in your area. if you have to have someone do all the work, then figure a minimum of about $700 to have the job done, again depending on labor rates in your area.
I have built 2 8.8s for my 67. It is pretty easy. An angle grinder and mig welder are the only tools I used other than tools to remove and install the rear end. If I was doing a 64-66 I would probably do an Explorer with 2 short side axles. I think it ends up being 3/4" narrower than stock at 56.5" wheel to wheel surface. It is pretty easy to find them at a pull a part with disc, 31spl axles and trac-loc. That length would work good with 14" or 15" wheels. If you want to run 17" or bigger wheels an SN95 length may work better. They are 2" wider than stock and make rear spacing similar to late model wheels making it easier to get wheels to fit w/o spacers. Make sure you know what direction you are going so you can get a wheel you like to fit with little or no spacer.