if you want it to handle, first thing, dump the big block and go to an aluminum headed small block or a genIII hemi.
firm feel makes upgraded T-bars for the F/M/J's now. even without them, my 5th ave handles very adeptly with cop spec rear springs, cop spec front sway bar, and a 1" rear bar....
as far as the Panther (crown vic/grand marquis) to F/M/J, the widths are a lot more different than you realize.
if you park an F/M/J next to a late model panther, it is quite evident. quick search on the intertubes show F/M/J's in the 72-74" overall width department, late model crown vics are more than 78".
if you look at the F/M/J's, the front suspension is ~ 61-62" wheel mount face to wheel mount face. the crown vic is more like 67". but the track is only 2" different you say? well, the F/M/J's use a ~+6mm offset wheel (80's M's use 15x7's with 4.25" BS, which is +6.35mm offset. crown 04-up crown vics use a +46mm offset rim....so you get over of 3" of track width reduction by wheel offset on the crown vic already. good luck trying to find a +70mm or so rim....
now this swap (at least the subframe) might be the cat's meow for a late B or a C body, which are significantly wider than an F/M/J.....
I think you'd be better off and cheaper in the long run building a custom chassis, or using an alterKtion, or looking for a wrecked vette and stretching that frame to fit, or sectioning the body to fit...a C5 (97-05) wheelbase is 104.5", a C6(06-present) is ~106". overall width for the vette's are ~73".
C5s/C6's also weigh ~3300 lbs, so stock spring rates may be very well matched to an F body.....
