they did not put that shaker on a 390 as far as I know unless he bought it aftermarket. it all looks right with the s-tube and all to be a 428 cobra jet so I would check the VIN. all motors were available for all cars back then. punch your VIN in here and it will tell you for sure

polution was the first thing people removed on these cars, more rare that someone added a shaker than removed the polution. a quick way to tell is there will be plugs in the heads above the exhaust ports

http://www.mustangdecoder.com/plates_1968.html

if its a cobra jet it would be worth a full restore. unrestored I bet you could get 20-30K

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1982 300CD Turbo (Otis, "ups & downs") parts for sale
2003 TJ with Hemi (to go anywhere, quickly) sold
2001 Excursion Powerstroke (to go dependably)
1970 Mustang 428SCJ (to go fast)
1962 Corvette LS1 (to go in style)
2001 Schwinn Grape Krate 10spd (if all else fails)