iron block/ iron heads. the last generation of the big block. I think '06 was the last year they were produced in 3/4 ton burbs. You could probably ge them in '07 classic HD's. Here's everything you probably would want to know from the reliable source wiki

Vortec 8100[edit]
L18[edit]
The Vortec 8100 L18 was a V8 truck engine. It was a redesigned Chevrolet Big-Block engine and was introduced with the 2001 full-size pickup trucks. It had an all-iron engine (block and heads) with two valves per cylinder. It retained the same bore diameter as the old 7.4 L Big-Blocks, but the stroke was upped by 9.4 mm (0.37 in) to reach 8.1L (496cuin), for a total of 107.95 mm (4.25 in) bore and 111 mm (4.37 in) stroke. Power output ranged from 340 hp (254 kW) to 550 hp (410 kW),[citation needed] and torque from 455 lb·ft (617 N·m) to 690 lb·ft (936 N·m).

Other important differences between the Vortec 8100 and older Big Blocks include a changed firing order (1-8-7-2-6-5-4-3), a new 18-bolt head bolt pattern, longer connecting rods, different symmetrical intake ports, different oil pan rails and the use of metric threads throughout the engine. The fuel-injection system for the Vortec 8100 is nearly identical to that used on Gen III engines, right down to the fuel and spark tables in the ECU.[19]

Vortec 8100s were built at GM's Tonawanda Engine plant. The last L18 was manufactured in December 2009.

L18 applications[edit]
The Vortec 8100 was the engine used in the largest U-haul, their 26-foot (7.9 m) truck. GM also sold the Vortec 8100 to Workhorse (now a division of Navistar), making it one of the most popular engine choices in gas powered Class A motorhomes during the first decade of this century. GM stopped installing Big Block V-8's in the Silverado HD trucks when the GMT-800 series was discontinued in 2007.

Chevrolet Silverado/GMC Sierra 2500HD & 3500HD (option)
Chevrolet Suburban/GMC Yukon XL (option)
Chevrolet Express 3/4 or 1 ton
Chevrolet Avalanche 2500 (std on GMT800 2500)
Chevrolet Kodiak
Workhorse Class A motorhomes.
T-98 Kombat armored vehicles.
U-Haul's 26-foot (7.9 m) truck.
Malibu Boats, e.g. Wakesetter.[20]