Your two potential boat choices are so vastly different, any comparison would be a WAG. One thing is certain, large boats with gas engines will burn lots of fuel at anything above hull speed
I have a pair of Marine
Power EFI 350's in my 28' GlasPly. A friend with a similar hull
powered with a pair of roller-cam, carbed, 350 Crusaders sees about 1.1 to 1.15 mpg on his Flo-Scans cruising at 30 mph at about 3500 rpm His boat loaded for cruising weighs 11,500 lbs. Another friend with the same hull
powered by a single
502 MPI gets about 1.2 - 1.3 mpg at 20 knots. At hull speed
he claims 2.75 -3.25 mpg. Better mpg at hull speed than the twin engined versions but that's probably a result of less underwater drag.
You pose a difficult question because there are so many variables. A 350 Crusader block is the same as a 350 Mercruiser and Marine
Power - all Chevrolet based.
I would say if you get 1.0 mpg on the Silverton you will be doing well.