R2/4J MC18 - 96mm crank webs, center seal next to middle bearing.
R5K MC18 - 96mm crank webs, integrated center seal into middle bearing, same as 21/28.
MC21/28 - 100mm crank webs, integrated center seal into middle bearing.

R2/4J cranks can only be used in R2/4J casings.
R5K cranks can be used in R5K casings and MC21/28 but not advisable.
MC21/28 cranks can be used in MC21/28 casings only.

As for your question what you would need to put the 28 crank(100mm) into your R5K casings, you need MC21/28 casings as the crank webs is too wide for the R5K casings(96mm). You can still use the same flywheel and pickups.

Cheers
Mario