J-Car or Mark I ?

You are confusing the two. The J-Car was the failed 1966 attempt at replacing the Mk II Ford GTs. It shared the engine and drivetrain with the Mark II and nothing else. You are not conveeting the Trumpeter kit to a J-Car, you'd be scratchbuilding the J-Car, stealing the drivetrain, itres, and miscelaneous bits from the Mk II.

The Mk I shares the basoc chassis and cabin with the Mk II. The 427 intrudes into the firewall which is entended into the cockpit. You would need entirely new front and rear clips and the rsdiator ducting on the Mk I would have to be scratchbuilt. Depending on the exact variant of the Mk i, the Mk II wheels would work, but would be a bit too wide. The engine and transaxle are totally different.