Can I do a direct swap of a 2000 ej251sohc into a 1999 Subaru Outback Limited with ej25d dohc?

no, not an easy direct swap.

your best solution using those two engines is to use the 00 block and install the 99 heads, intake, etc.

and then install it in the 99 outback.

 

or find an ej25D 96 - 99 (NOT the 99 forester)

or an ej22 95 - 98 and use that .

Thank you for the answer. I searched the old posts, but couldn't figure it out.