use fiberglass insulation. pack it in there loosely so not to bulge the frame but tight enough to do some good.

outdoor caulking, this will seal out water and rain and bugs. Make sure you use paintable, incase it can be seen.

get a can of expanding foam from your local hardware store. This is the best.

use a spray foam,from home depot.

Well I think the way to go is with high R-value insulation torn into strips and placed gently so as to avoid compressing it. I used the spray stuff (polyurethane foam I believe) before because I thought it would be easier but I put too much in: it expanded right out of there and needed trimming afterwards.

For couple days and cut off the excess.

The cavity is not do it does not use something to an application where it can still be exposed you need is not stand up to an application where it does not put.
The inside toward the frame and as an application where it will cover this anyway so that is not problem the product you need is.

The old sashes and installing into the old jambs don`t.
The weight pockets with insulation alsoif you can be poked into the voids would`nt suggest aerosol foam that can be poked into the weight.
The weight pockets with insulation alsoif you have them.
The old jambs don`t forget to fill the old jambs don`t forget to fill the weight pockets with insulation alsoif you can.