ACR CLRED FOR VISUAL NOT TO PASS ADJACENT ACR BUT UNABLE TO STAY BEHIND WHEN ADJACENT ACR SLOWED.

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FLT X INBOUND TO SEAL BEACH CROSS AT 6000 FT WITH SO-CAL APCH. AFTER SEAL BEACH VECTORED 350 DEGS; RECEIVED LOWER ALT. THEN VECTORED 280 DEGS FOR LOC INTERCEPT. INTERCEPT APPROX 12 MI OUT; STILL ABOVE GS. TFC FOR RWY 25R (ACR Y) GIVEN AND SIGHTED. WE WERE FOR RWY 25L. TOLD TO 'FOLLOW ACR Y' STAY BEHIND HIM; 'KEEP THE SPD UP' 180 KTS OR BETTER' TO LIMA (FAF) CLRED APCH RWY 25L. INITIAL SPACING APPROX 1 MI. STILL SLIGHTLY HIGH ON GS. SPD 180 KTS; FLAPS 20 AND GEAR EXTENDED BY MARKER. GS INTERCEPT JUST INSIDE MARKER WITH SLIGHT CLOSURE IN ACR Y. BEGAN FINAL SPD REDUCTION; IDLE THRUST; LNDG FLAPS; STILL SLIGHT CLOSURE RATE; GOT TCASII 'RA' -- TURNED OFF TCASII TO SILENCE WARNING. SPD STABLE AT 157 KTS; ON LOC AND GS APPROX 2 MI INSIDE MARKER; SEPARATION APPEARED CONSTANT (APCH 1/2-3/4 MI) THROUGH '1000 FT' AND JUST PRIOR TO '500 FT' WARNING ACR Y APPEARED TO RAPIDLY DECELERATE. WE INITIATED FURTHER SPD REDUCTION TO 150 KTS (MINIMUM APCH) UNABLE TO STAY BEHIND ACR Y AND PASSED JUST BELOW 500 FT. I KNOW WE WERE AHEAD OF AND SLIGHTLY BELOW. THE RWY WAS ASSURED AND I RECEIVED A SAFE LNDG WAS POSSIBLE FOR BOTH ACFT AND MADE DECISION TO CONTINUE FOR AN UNEVENTFUL LNDG. PROBLEM?: TECHNICALLY; ONCE WE PASSED ACR Y AND LOST VISUAL CONTACT; WE COULD NO LONGER 'MAINTAIN VISUAL SEPARATION' AND I BELIEVE WE WERE ILLEGAL TO CONTINUE. HOWEVER; AT THAT POINT; A 'GA' WOULD HAVE ALSO LEFT ACR Y BEHIND US AND OUT OF SIGHT (ONLY BELOW; INSTEAD OF SLIGHTLY ABOVE OUR ACFT) AND THAT DID NOT SEEM PRUDENT. HOW COULD THIS HAVE BEEN AVOIDED?: 1) XING SEAL BEACH AT 6000 FT ALREADY PUTS US VERY HIGH ON THE INITIAL APCH SEGMENT FOR A MASSIVE ACFT LIKE THE 747-400 (I UNDERSTAND THIS IS BEING DEBATED WITH THE FAA) AND BEING INSTRUCTED TO KEEP 'OUR SPD UP' FURTHER COMPOUNDS THE PROB (WE CALL IT THE 'SLAM DUNK'). I THINK WE COULD HAVE BEEN MORE ADAMANT ABOUT GETTING LOWER AND REDUCED TO FINAL SPD SOONER. 2) ATC IS VERY UNREALISTIC AT TIMES WITH XING ALT; SPD AND SEPARATION REGARDING 'HVYS' AND OTHER ACFT. THIS NEEDS TO BE ADDRESSED. OUR AGL SPD AND ACR Y APCH SPD WOULD OBVIOUSLY BE VERY DIFFERENT (OURS BEING MUCH HIGHER). 3) FINALLY; THE ACR Y DID APPEAR TO MAKE A LARGE AND FINAL SPD ADJUSTMENT WHEN INSIDE THE MARKER; PERHAPS NOT GOOD PLANNING ON THEIR PART EITHER. IT CAUGHT US TOTALLY BY SURPRISE.