Not sure if this will clarify anything but this chart on engine weights is pretty interesting. I pasted part of it below. Looks like an iron FE is about 125 lbs more than an iron 351W.
http://fixrambler.com/engineweightchart.txt
Ford BOSS 302 500 (48)
Ford 351 Cleveland 550 (48) (includes BOSS and Australian 302-C)
Ford 351 Windsor 510
Ford 351 Windsor 525 (48) (168)
Ford 351M-400 575 (48)
Ford Y block V8 625 (272-312 CID)
Ford FE big block 650 (332-428 CID)
Ford FE big block 670 (1) ('59 352 CID)
Ford FE 625 (48)
Ford 427 SOHC 680 (48)
Ford 429/460 V8 640
Ford 429-460 720 (48)
Ford 460 V8 720 (10)
Ford BOSS 429 680 (iron block, aluminum heads)
Ford BOSS 429 635 (48)
Ford 4.6 SOHC 530 (93) iron block, aluminum heads
Ford 4.6 SOHC 473 (72)
Ford 4.6 SOHC 600 (168) (Mustang)
Ford 4.6 DOHC 464 (72) "9 pounds lighter than SOHC"
Ford 4.6 DOHC 437 (77) without accessories
Ford 4.6 DOHC 521 (94) aluminum block and heads
Ford 4.6 DOHC 576 (168) (Mustang)
Ford 4.6L (SOHC) 529 (231) w/flywheel 1998 Mustang, dressed (dry)
Ford 4.6L (SOHC) 492 (231) w/flexplate 1998 Mustang, dressed (dry)
Ford 4.6L (DOHC) 535 (231) 1998 Mustang, dressed (dry)
Ford Taurus SHO 3.2 V8 390 (229) Taurus SHO V8, no accessories