A few very important things must be changed to swap a newer 5.0L HO motor into a Mustang II.

1. The 5.0L HO uses a 50oz imbalance instead of 28oz imbalance for it's external balancing.

   1. You must have the flywheel or flexplate balanced for a 50oz imbalance instead of the stock 28oz imbalance.  

   2. You could use a newer flywheel or flexplate, but the bellhousing must match.  The Mustang II bellhousing is smaller diameter than most bell housings and requires a special sized flywheel and flexplate.

   3. B&M has a correctly sized flexplate available with the 50oz imbalance.

   
   
   

2. The 5.0L HO has a Roller cam made of billet steel instead of cast iron.  You must use a steel or bronze distributor gear with the roller cam.  The stock Mustang II distributor has a cast gear.

   1. If you're going to use EFI, the stock distributor already has a 0.531" inside diameter steel gear.

   2. The Mustang II distributor for DuraSpark uses a 0.467" inside diameter distributor gear.  Aftermarket bronze or steel gears are available from Mallory and MSD.

   3. The '82 DuraSpark distributor will fit a 0.531" inside diameter distributor gear so you  you can use the stock replacement gear from a roller cammed 5.0L HO

   
   
   

3. The 5.0L HO aluminum valve covers can be used:

   1. To clear the Mustang II air cleaner snorkel, they must be installed on the opposite sides from stock so the oil filler neck will not be in the way.

   2. The PCV hookup is only 3/8" instead of the normal 1/2" so either a hose adapter must be used, or a PCV valve from a different application.  I used an '80s Nissan screw-in PCV valve, screwed directly into the intake manifold with a 3/8" hose from the valve to the oil filler neck.

   3. The crankcase must also have a free flowing vent.  The EFI intake has a large vent from the crankcase so it doesn't have any vents in the valve covers.  To resolve this you need to replace the oil filler cap with a screw on K&N breather.

   
   
   

4. The 5.0L HO uses a serpentine belt system.  If you want to retain the Mustang II belts and accessories, the water pump must be replaced with an older non-serpentine water pump because the serpentine water pump spins backwards.





5. The 5.0L HO uses a 351W firing order.  Change the firing order to 13726548.





6. If you plan on leaving the Mustang II air pump installed, it bolts directly on.  If you want to remove it, install two 1/2" plugs in the air port inlets at the back of the heads.





7. The 5.0L HO uses an electric fuel pump so there is no fuel pump eccentric or fuel pump mount in the front cover.  Either swap the eccentric and front cover from an older 302, or switch to an electric fuel pump.





8. The 5.0L HO uses a dual hump, rear sump oil pan that will not fit in a Mustang II.

   1. If you're using a Mustang II bell housing, a Mustang II oil pan will need to be used, otherwise any front sump pan will fit.

   2. The dipstick needs to be moved to the front of the motor.  The 5.0L front cover would need drilled to accept the dipstick, or an older 302 front cover will already have the dipstick hole.  Don't forget to plug the dipstick hole in the block.