WAKE TURB -- ACR CREW IN A B727-200 ENCOUNTERS THE WAKE FROM A DC-8 DURING TOUCHDOWN.

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FOLLOWED A DC8 TO RWY 9L AT MIA. STAYED 1/2 DOT HIGH ON GS TO AVOID VORTEX. DURING FLARE EXPERIENCED AN UNCOMMANDED ABRUPT R ROLL. R TRUCK STRUCK THE RWY IN SPITE OF ATTEMPT TO STOP THE ROLL WITH PWR AND CTLS. SUBSEQUENT TOUCHDOWN WAS NORMAL. THIS LNDG WAS DEBRIEFED BY TELEPHONE WITH MIA TWR AND MIA FLT OFFICE. CALLBACK CONVERSATION WITH REPORTER REVEALED THE FOLLOWING: THE RPTR WAS ORIGINALLY ASSIGNED RWY 9R IN MIA; BUT HE REQUESTED RWY 9L WHICH IS CLOSER TO HIS AIRLINE'S GATES. HE WAS FLYING A B757-200 AND THE PRECEDING ACFT WAS A DC- 8 ABOUT 3 MI IN FRONT. THE RPTR WAS NOT TOO CONCERNED BECAUSE HE THOUGHT THAT THE 17 KT XWIND WOULD BLOW THE WAKE AWAY. HOWEVER; THE DC-8 SLOWED MUCH MORE THAN HE EXPECTED; AND AT ABOUT 300 FT ABOVE TOUCHDOWN; THE DC-8 WAS STILL ON THE RWY AND THE RPTR WAS CONSIDERING A GAR. DURING THE FLARE AT ABOUT 10 FT ABOVE THE RWY THE ACFT SUDDENLY ROLLED TO THE R AND MOVED TO THE L LATERALLY AND TOUCHED DOWN FIRMLY. THE ACFT HDG DID NOT CHANGE DURING THIS MANEUVER. THE ACFT DID BOUNCE; BUT NOT HIGH; AND TOUCHED DOWN AGAIN MUCH MORE SOFTLY. THE RPTR DID ADMIT THAT HE WAS AT ONLY VREF PLUS 5 KTS AND NOT AT PLUS 10 KTS; WHICH WOULD HAVE BEEN MORE CORRECT BASED ON THE WIND CONDITIONS AT THE TIME. THE RPTR SAID THAT IN THE FUTURE HE WILL MAINTAIN AT LEAST 3 MI IN TRAIL AND THAT HE WOULD NOT BE SO EAGER TO SWITCH RWYS TO FOLLOW ANOTHER HVY ACFT LIKE THIS. HE SAID THAT IN A POST FLT INTERVIEW WITH A TWR SUPVR HE ASSURED THE SUPVR THAT THE CTLRS WERE VERY PROFESSIONAL AND THAT NONE OF THE EVENTS WERE THEIR FAULT. THAT HE SHOULD HAVE FOLLOWED THE CTLRS ORIGINAL ASSIGNMENT WAS A CONSISTENT THEME THROUGHOUT THE INTERVIEW.