Abused capping

Strange how that happens is'nt it. Anyway if you should decide to install windows from the exterior you will have to re-cap, due to the removal of the outside stop. When we do that we cut it off with a sawz-all retaining the stop as it will be an exact fit back on, then bend new capp that will float over top of the stop and return back to the vinyl, as tightly as possible, then you should have 2 pretty good surfaces (al,vinyl) to chaulk to.
Also inspect the sill very good. If need be cut of the first couple of inches usually even with the exterior wall sheeting, then rip a new piece of treated 2by lumbers down using somewhere around a 10 degree angle to give the new sill positive slope outward, just face screw it in the existing sill, then cap sill running the AL as far in to interior as you can get (at least far enough that the replacement will sit on it, then install the replacement. You will probally have to finish nail a piece of parting stop on the sill at the bottom of the new replacement due to the sloped sill of the orginal window(the new one won't be made with a slope on the exterior or bottom of the window) then just bend a 3/4x1/2 90degree angle to cap it. Again you'll have the best surfaces to chaulk to.
Be sure to get some 3.5" x 1" window insulation to stuff around the replacement and old wood frame. A 3" putty knife works well for doing it. BTY the replacement should be made 1/2" smaller in width and height to accomadate to this insul. Don't forget to insulate under replacement also, just lay a piece in maybe staple it down so it can't move out on ya, then tip the replacement up in to the hole. Also be carefull where you screw the window into the old frame, you don't want the screw head interfearing with any sliding action of the sashs, Laugh if you will, but it happens. If they are real wide like 48" then you'll have to run a screw though the sill

. Most replacement have air chambers so yo can carefully drill a hole though the first layer of the sill, they same size as the head, then run your screw down though the hole it should go though at least a couple more air chambers so the head will catch on the second layer, then CHAULK the screw head. Then if you can find some plastic plugs you can just plug the visible hole. We use the plugs that you would find in a door's glass frame to hide there screws. Anyway this should be done as a LAST RESORT to get the bow out of the sill.
Just some tips. Any ? please ask