The Academy kit's fit assembly is very easy with good fit and clever engineering on the many parts. The gaps are mostly covered by next to be assembled parts so they won't be seen.

The main bay is seen here with some added detail from sprue, but actually almost everything is already there with the kit parts.

Some attention is needed at the LEX'es. Sand and fill.

Also, the intake parts need some carefully aligning and some filling. It was decided not the put stress onto the assembly, a small piece of card will close the small gap nicely. Some putty to be added also later on.

The wing of the Academy kit warps a little at the outher tips. Recently someone told me the following about this:
The outer wings of the real F/A-18 are slightly "curled" up. This has been done to solve some "flutter-problems" on the actual aircraft. This has been done also in order to achieve a better aerodynamic release of the AIM-9 sidewinder missile, normally carried on the outboard stations.. so ACADEMY actually has got it right!!!

Furthermore the outer wings are slightly "tilted" downwards when viewed "head-on", that is to say standing in front of the aircraft. This is the outher wing dihedral. Assure and check that the wing on left and right sides is symmetrical. 

The Academy kit can be used as a "reference" as this kit is very good and accurate.

First, the Hasegawa kit undercarriage doors can be detailed on the inside with some card. The Academy kit was used as pattern / example. Some is true for the main undercarriage legs.Some pieces of card and stretched sprue does the trick. Gaps can be closed with white glue and you get a smooth part.

The forward fuselage nose section looks of the Hasegawa kit can be improved in detail by using the extra part # E14 of the Academy kit as it has nice grilles and vents. It fits good after removal of the white lower section of plastic with some putty and sanding. 

The nose bay was also detailed of both kits

 Aft of the ejection seat, a fine grill ("screen") covering the F-18 avionics compartment was to be added. The gap was made in the parts of both kits where the grill is and the bay will be added later. The fine mesh/wire is made of an old girl's panty, stretched and glued in place with superglue. It will add further realism to the model.
Below the screen a number of avionics boxes will be made, but they are hardly seen.

The Hasegawa kit's wingtips will be set folded using parts of the Academy kit. The Academy kit will be assembled in the "clean mode" with no folded wings, no folded radar cone etc.
The extra fairing parts #D30- and # D31 are to be added to the Hasegawa kits' lower wing after sanding it off. 

The bare Hasegawa cockpit area needing some work...

The main bay was deepened and detailed. The flap edge was filled and set /rescribed in a more correct forward position. 

The lower fuselage area at the nose with the extra Academy part #E14  in place.

On to next page [3]......