When a 351 is installed in a First Gen Mustang, its pretty much a bolt in (If the car had a V8 to start with). Any parts vendor that deals with mostly antique stangs will be able to head you in the right direction.
I do believe you have to get a certian type of header for Steering clearance reasons.
As for the diffrence between 6cyl and 8 cyl steering pieces, there are none. I'm pretty darn sure (ALthough I am wrong alot  ) there the same as the V8 pieces.
The V8 motor mounts will work, nothin special there.
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64 1/2 "D" code Red Mustang Coupe.
Red Baron,
2000 Perf Red Mustang GT. 5spd. Stage 2 CNC heads with Large Valves, Comp Cams (270-274 dur .550 lift), Forged Crank, DRP pistons, Manly H-beams, Pro M 87 mm Mass Air, 24lb Inj, 75mm TB, Plenum Spacer ,Long Tube Headers, Homemade H-pipe, 2 chamber Flows, 4:10's, Steeda Tri-Ax, LKW 70/30's, Eibach Drag Springs, Steeda SubFrames, Battle Boxes, Rear Girdle, Adj uppers, Alunimum Drshft, Southside Lift Bars.
Project Cars, 64 1/2 6cyl coupe, 1930 Model A Nostalgia Rod.
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