P71 to Grand Marquis Engine Swap Tips, and general diagnosis for engine issue

Okay, this is technically a two parter. They tie together at the end so just hear me out on this.

I have a 2003 Mercury Grand Marquis LS with 199k miles on the engine. It's interior is good, body is sound, and has had regular maintenance since I have owned it. Shortly before I parked the Grand Marquis in a garage, I noticed that it started having intermittent starting problems. The Grand Marquis would run perfectly fine, and then, after parking it for any amount of time, when I would get back into the car it would not turn over. All of the electronics would work fine, no code was being shown, and it would make a kind of whining or whizzing sound while attempting to turn the engine over. This would happen with both hot and cold engines and for different amounts of time (5 minute upwards to 45 minutes) before starting up again. I had two mechanics look at it and both put reman'd starters in which did not help. I even captured the event on camera to show the mechanics as to not seem completely insane trying to explain the issue to them. Due to this issue, I parked the car in an enclosed garage for safe keeping, and at this point needs more work from sitting to fire up.

Recently I got my hands on a 2007 P71 Crown Vic with 97k miles on the engine. I like the look of my Grand marquis better (especially the interior, though I have driven p71 crown vics by trade for a long time and am used to the cloth interior), but wanted to try and see about taking the 4.6 out of the P71 and dropping it into my Grand Marquis. I am aware that P71's are not limited and tend to top out at about 130 compared to limited around 120 mph. I was wondering if anyone had any tips on if I should take the P71 engine and drop it in the Grand Marquis, take the Grand Marquis interior and drop it into the P71, or it anyone has any other (helpful) ideas.

I love my panther bodies, and any help with this conundrum of mine is welcomed.

Thank you in advance.

On the starting problem: the whining or whizzing when you try to crank is the key clue. That's the starter motor spinning without turning the engine. Two starters didn't fix it, so look at what the starter gear meshes with, the ring gear on the flexplate. When the engine stops at the same few spots, those teeth wear or chip, and the starter spins against the damaged section until the engine happens to stop somewhere else. Pull the starter and inspect the teeth through the opening, turning the engine by hand. A worn flexplate is a lot less work than an engine swap.

On the swap: both are 4.6 engines from the same family, so it can be done, but match the computer and wiring to the engine, and fix the flexplate first. You may find the Grand Marquis doesn't need a new engine.

Dana