Can't compare weight to a red block....never had a red block to weigh. But overall, the car is within the range of curb weights listed for 242's through the years. The 5.0L that came out of the car (with aluminum heads) weighed 411 lbs. The LS3 that went in weighed 393 lbs. No flywheel on either, no accessories. With heads, valve covers, intake mani, fuel and ignition systems, shorty tubular headers on the 5.0L and cast iron manifolds on the LS3. Of course, the flywheel, clutch ass'y, bell housing, and tranny are all a bit heavier for the V8 than for the 4 banger. All in - I'd guess 75-100 lbs. more for the all aluminum V8 compared to a red block. But with the battery moved to the trunk, and the Commando bumpers laid up in fiberglass (that knocked about 50 lbs. off the car) - it all comes out in the wash.

Keep in mind - the 5.0L Ford motor has a deck height of 8.2"; the LS3 is 9.2" - same as a 351 Cleveland. Those typically weigh about 75-100 lbs. more than the 5.0L's. So that gives you another reference point. An efi 351C with aluminum heads/intake would likely weigh 100-120 lbs. more than the all aluminum LS3 in the car.