Where do you get your numbers? You don't need to do forged anything to upgrade to turbo.
The engines are pretty much the same with some minor ones.
To turbo any Z (70-83) all it takes is the turbo, oil feed line, manifold, j-pipe (or FMIC), EFI intake, and either the stock wiring harness or an aftermarket FMU.
I did this exact same swap and spent about $700. $700 < 6k+
The second best swap price wise is the SBC or small block Ford. When I'm done with my v8 swap i'll have less than 2k into it.
The most expenisive is going to be the RB series.
For example take the RB25DET NEO
Front clip - $1500-$2500
FMIC setup - $400
Special intercooler with brackets - $400
Engine/tranny mounts -$600
Rear sump oil pan (custom job) - $500
Wiring harness revamp - $300-$600
Drive shaft - $200
Exhaust system - $300-$800 depending on how you want it to go
Throttle cable -$70 (lokar)
Hoses, fueses, extra wire & connectors, fluids ext... say about $200 or so
So say you get a good deal on the front clip you'll be spending close to anywhere from 5k-10k just for just parts not including labor and beer for when you mess something up.
I digress yes the RB will fit in a z/zx but it will not be cheap.