Not much is usable off it. You are pretty much going to spend more money searching for all the odds and ends you will need. The ECU is batch-fire, and the wiring is speed density as well. Injector harness is pinned incorrectly for the non-HO firing order. The heads are E6SE heads, the intake and Tb are the early 1986-style Mustang manifold which is tiny. Cam firing order is the non-HO order.
I wouldn't buy the car for a 5.0 swap. Buy yourself a 96-01 Ford explorer/mountaineer 5.0 engine and then find a clean 4-cyl and do a carb swap setup, or if you want EFI you'll have to search down the expensive 5.0 Mustang ecu and wiring.
However, If you already have a 1998 GT, just do a 99-04 PI head/cam/intake swap to that car, with an aftermarket plenum and TB and full exhaust and you'll be at the 300 crank HP range anyway. I don't even know why you'd bother with a 5.0 pushrod swap unless you are swapping in an AL-headed 347 or something like that. You can probably buy the PI swap parts for about what you'd pay for the marquis.
PI swapped '98 2V 4.6L > Gran Marquis 302 engine