Would a 8.1 liter chevy big block be considered a engine swap in a car or truck that original came with a 396-454 big block?
My understanding is that the heads will not interchange without modification. Also most of the exterior(front cover, oil pan) parts will not interchange. Same bore spacing, Cranks I believe will interchange.
From Wikipedia
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.