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.