So, I too have a 3.5" tft screen which uses this MCUFRIEND.h, running on a MEGA2560. Plugged in as a shield, the unit takes D2-D9.
God invented Shields. They make correct and reliable electrical connections.
Your Shield is designed for Uno but will "work" on MEGA2560. The Mega will be slower than the Uno because the Shield pins are connected to random Port pins on a Mega.
Yes, you can make an Adapter Shield by hand-wiring a Mega ProtoShield. Routeing the data bus to D22-D29. Routeing the microSD to D50-D52.
The SPECIAL has been in MCUFRIEND_kbv forever. You just enable it by removing the // comment characters in two files.
David.