I mentioned the Vega engine to a mate that works in engine development and he found it hard to believe they went all that trouble developing the block process then put an iron head on it to save money but then maybe that was GMs way. I do remember back in the 70s the engine had a bad reputation.
IIRC one of the Mercedes Benz engines is the same, a V8 I think / /
I looked up what HIP was and I think you're right that wasn't what was used rather heating to a high temp and striking the powder pressing to consolidate the material. I noticed a youtube video on HIP so will watch that shortly.