Here are some interesting specs from Car Life magazine, May 1966.
1966 Specifications. Included are the engine weights of American cars. Some are listed without the flywheel, so those will be noted below...
Engine------------------------ Weight
AMC 327-cu-in V-8------------ 600 lbs
Mopar 170 6-cyl--------------- 465
------ 225 6-cyl--------------- 500
------ 273 V-8---------------- 550
------ 318 V-8---------------- 605
------ 361 V-8---------------- 640
------ 383 V-8---------------- 650
------ 426-Hemi--------------- 725
------ 440 V-8---------------- 685
Buick 300 V-8----------------- 470 (no flywheel)
----- 340 V-8----------------- 500 (no flywheel)
----- 401 V-8----------------- 640
----- 425 V-8----------------- 650
Cadillac 429 V-8--------------- 600 (no flywheel)
Chevrolet 230 6-cyl------------ 465
--------- 250 6-cyl------------ 480
--------- 283 V-8 2-bbl-------- 575
--------- 283 V-8 4 bbl-------- 585
--------- 327 V-8------------- 610
--------- 396 V-8------------- 687
--------- 427 V-8------------- 700
Oldsmobile 330 V-8------------ 570
---------- 400 V-8 4-bbl------ 610 (no flywheel)
---------- 425 V-8 2-bbl------ 620 (no flywheel)
---------- 425 V-8 4-bbl------ 630 (no flywheel)
Pontiac 230 6-cyl 2-bbl-------- 490
------- 230 6-cyl 4-bbl-------- 500
------- 326 V-8--------------- 600 (no flywheel)
------- 389 V-8--------------- 675
------- 421 V-8--------------- 685
Ford 170 6-cyl---------------- 356
---- 200 6-cyl---------------- 380
---- 240 6-cyl---------------- 450
---- 289 V-8----------------- 485
---- 352 V-8----------------- 647
---- 390 V-8----------------- 660
---- 410 V-8----------------- 670
---- 427 V-8----------------- 670
---- 428 V-8----------------- 680
---- 462 V-8----------------- 735
It looks like the Lincoln 462-cu-in is the heaviest of the listed engines at 735 lbs with the Mopar 426-Hemi close behind at 725 lbs.