Gas guzzling V8's...maybe a big block could be called that, but I have seen and owned many small block ford V8's that can get close to 30mpg, and still run carbs and have 3spd Auto fitted in cars that weight about 1400kg.
My father owns a 1979 Ford XD Fairmont ESP sedan fitted with a 302cu(5litre) Cleveland V8,C4 3sp Auto and 3.25:1 9inch diff, it produces 380hp at the treads and when driven sensibly will give 28mpg on the highway and 20mpg around town.
I own a 1986 Ford XF Falcon S fitted with a 250cu(4.1litre)inline 6 with Multipoint EFI, 5spd Manual and 2.77:1 diff, it produces maybe 150hp at the treads(190hp at the flywheel) and on a good day it might give 25mpg on the highway and about 20mpg around town.
both cars weigh around 1400kg and have a similar body shape(the XF is an update of the original XD(code name Blackwood)bodyshape.
the term Gas Guzzling is an out dated term, these days most EFI V8's can give economy figures similar to 6 cylinder cars....when driven with economy in mind...you have no hope if you drive them hard!