2.4L mercs were the lightest (Late 70's 339lbs)
Mid - 80's 2.4L 200's were 363lbs
92 - later 2.5L (150 - 260) were about 390.
I don't have the specs on the race motors
245, 260, 280 . . . They had a different trim setup with the HUGE single ram pistons like the 84 and earlier Mercs with an external pump.
After messing around with the motor height . . . I can just touch 50mph with my seafari and an old old Merc Vertical Reed 2.4L 200 (really 175ish) with a 19Pitch Mirage plus.
2.5L Mercs are pound for pound the meanest motor out there. These motors have a LOT of bottom end torque and they pull HARD all the way up to WOT. I like the 2.4's b/c of the economy, chrome bore cylinders, and light weight. a 2.4L 200 has better fuel economy than the 2.5L 150 EFI Or Carb.