I also got me a 1989 Grand Marquis 5.0 engine and got it installed in a 66 coupe. This is what I did.
1) Re-used the oil pan and pick up tube from the 66 289 and use a single piece oil pan gasket( Front vs. Rear oil sump)
2) Get a harmonic balancer from summit 50 oz balance that will take 3 or 4 bolt pulleys with V-Belts
3) Get a steel distributor gear for the 289 distributor (steel cuz the roller cam needs it)
4) Swap the timing chain cover from the 66 and while at it change the roller chain and gears (Oil dip stick It’s at the front in timing chain cover vs. Side oil pan on the 5.0) Timing marks will be on the driver side vs. passenger side in the 5.0
5) Buy a 157 tooth 50 oz balance flex plate
6) Get Felpro gasket for the intake (Victor and a few others are a little to big in the front water jacket)
7) Buy 2 pcs of 9/16"-12 1 inch long Screws to plug the crossover tube at the rear of the heads.
The starter will bolt no problems, the engine mounts also are OK, and all my A/C, Pwr Steering and Alternator brackets fit right in. The Oil slinger in the Cam gear had to be massage to fit the double roller chain but that was a snap. Exhaust manifolds go right in and also headers, you will need the fuel pump and the pump cam if not using electric pump. If using electric pump get a cover plate for the timing chain cover. The water pump must be a standard rotation cast iron type. Also re-use the oil pan torque distribution bars on the 289 oil pan.
The 1989 5.0 i'ts a strong foundation for any build. First of all is a roller assy then is one of the strongest blocks from the 80's and is the same block as HO and others. Beside from the heads everything else it's fine. Just trow a set of ported E7TE heads, a b303 cam and 1.7 rockers from an SVO along with headers and you will be up in the numbers with a low cost build.
Here is a list of parts from summit:
SUM-163302 50 oz balancer
(2 pcs) 9/16-12 bolts to plug crossover tube at rear of heads
MAA-29418PD dist gear steel
TCI Auto 529610 - TCI Flex plates 50 oz 157 tooth
RNB-635-107 timing chain cover (optional)
fel-pro FPP-1250 intake gasket
I hope this helps.