If the heads are not warped, prep would be block sanding.

Would you sand your engine block gasket surfaces? - NASIOC

Would you sand your engine block gasket surfaces? Service & Maintenance

forums.nasioc.com

As for the engine block, toward the end of this article it implies the block should be prepped like the heads:
"... the surfaces for both the cylinder heads and engine block must be clean, smooth and free of any residue such as brake cleaner. But most importantly need to be at a refinishing average of RA 50 or better."
Subaru Head Gaskets Problems Explained Part II - All Wheel Drive Auto

Also, if you have not already removed the engine, you should consider it. This is said to be one of the most important factors in a long lasting repair, for improved access to surface prep and angle torquing.

https://www.subaruforester.org/threads/another-stinkin-head-gasket-thread.65495/page-2?post_id=739093#post-739093

 

Interesting point about the grounding straps and coolant pH.

 

@SgWarrior1 Are the valves, stems and seals in good condition? On my 2001, at 350,000km needed the heads sent off to have those issues fixed along with the machining the surface.

 

Good point. When I had my head gaskets replaced preemptively at 50,000 miles/8 years while the timing belt was being done, the shop found that a few of my exhaust valve guides were "dropping". The valve guides, press-fitted into the heads, were slipping down into the combustion chamber. If not addressed, this would eventually cause a problem.

The shop sent the heads out for repair:

Remanufactured cylinder heads (compl valve job) for dropped valve guides. Replaced cam seals.

1263.23​

 

thank you all who have replied and helped with my head gasket and other maintenance stuff i finally put it all back together today and it runs and drives amazingly, engine sounds good, small issue with a bad noise from the AC but apart from that it's great.

 

@Frizwin Don't know. The shop that did the R &R for the head gaskets sent the heads off to a machine shop. But keep in mind the heads on my 2001 had 350,000km so they were due.

 

I agree with you. if going through all that removal, and higher mileage, its worth the time to have head completely redone.