You are thinking about swapping a KA/SR tranny onto a L28. Doing it the opposite, you will have to fill it in, Or jam a brass bushing into the bell housing to tighten up the countershaft. I'm not sure as to which would work best, because I haven't do it before.
The fork won't be an issue, because Everything in front of the transmission has to match the bellhousing (SR bell housing)
So~, you will need a SR bell housing, flywheel, clutch kit, fork; L28 transmission. then swap bell housings.