Is it still the case that you don't need an IVA if you are using an original chassis, plus two of the engine, gearbox, steering or axles? More for idle curiosity rather than a plan for a new project. It seems odd that you're still allowed to do this.

Do you mean the 8 point rule? That is still valid.
But you cannot chop the chassis about....

The 8 point rule still applies and 5 points MUST come form the original and unmodified chassis.