It depends on what you want from Blaster Law - if you want to use the attack tables, then you need to map HARP SF armors to Spacemaster armor types and deduct the base armor DB out of DBs. I think you will increase lethality.
If you want the critical tables, then you could map Tiny to A etc., but you need to be alert to some Spacemaster crit effects don't exist in HARP SF.
Others have produced more elaborate conversions in the Fantasy domain, but I have not yet been convinced that any are the mechanism that I would pull into HARP officially to provide greater variety of combat results.
Best wishes,
Nicholas