But still, at the cost of all this, you may as well twin turbo the VQ35DE in the car. I'll stick to an RB26DETT for my example since an SR20DET doesn't have the torque off-boost to move a 1.5 ton car the way a sports coupe is meant to, and the Nissan V8s aren't as common in Australia.
Buying an R32 GT-R front cut will cost you around $6K, and you don't want a 15 year old engine so you're probably looking at a fair amount more. Lets say $8K for an R34 front cut. Otherwise you're comparing a brand new motor with a pretty tired one, and that's not particularly fair.
Just installing the block into the engine bay won't be cheap either since so much needs to be custom made (engine mounts, etc) that its going to take longer, and then you need to hope the gearbox can be mated or otherwise a fair amount of your driveline might need to be replaced too.
Lets be generous and say it'd cost $3K for a fully engineered install (considering the estimates for a VQ35DE -> DETT conversion by APS or an approved dealer is estimated at $3K to lift the engine, fit the parts, and then bolt it pretty much back into the factory spots) and you're looking at $12K for the motor without all the custom engine bay ancillaries your new motor will require. Since both need to fit intercoolers in places that didn't originally have them, we'll assume for this exercise that the cost of that cancels each other out (although, in reality, since APS has designed the thing to fit but the R32 intercooler parts you bought weren't, it will take more effort).
If your gearbox and driveline has to be replaced (which the American D1 drifter who did the SR20 conversion had to do), you're $15K will cover the parts but you've got nothing left for install and engineering.
Assuming the standard 'box works, at $12K you're now making approx 10% more power than if you'd just left the car alone. I've chosen this figure as its quite known that the GT-R's power output is underrated, and the estimates are that it puts out 310hp rather than 280, or 10% more).
Can you throw $3K into an RB26DETT and have it make more than 80% of its stock power while still having it engineered? Because that's pretty much what the APS TT kit is delivering over a stock 350Z (320rwkW over the 160rwkW stock power) for the cost of an RB26DETT conversion with light, bolt-on mods. And even then you're only just breaking even.
Bear in mind that at this cost you may not have a radiator since the GT-R nose is longer and the stock 350Z and GT-R items may not fit, there's no guarantee your Z33 air con will connect to the appropriate place on the RB26, and there's a whole host of other engine ancillaries I haven't included that need replacing or relocating for your differently-shaped motor. Paying for which will cut in to your $3K modification to up the motor from its standard 230kW to the 400kW the twin turbo kit delivers while still offering all these conveniences.
Its not really value for money, is it?
As unlikely as it looks from above, even if the RB26 can equal the VQ35DE+TT's power output for the same install cost, its not worth it since you've gone to all this effort, and replaced so much with items that don't have OEM build quality and tolerances, to get to the same place.
And since you're missing almost a litre of capacity with a far lower compression ratio, the APS VQ35DE+TT will deliver you much better response and midrange. In commuting, they'll probably have equal fuel economy due to the fact that you can drive it off boost pretty much all the time and the engine is more efficient with the higher compression.
Being cast iron, the RB26DETT won't be that much lighter (if at all) than the all-alloy VQ35DE. While this means it might handle more power than the VQ if you go for big numbers, for light tune applications there's no weight benefit but there is a weight distribution penalty. Also, the RB26 is still no 2JZ when it comes to block / bottom end strength.
If your budget extends past the $15K to further to truly exploit the power of the RB26 after install, you could rebuild the weak bottom end of the VQ35 and the APS boys will sort you right out
Of course, the RB is more of a known quantity than the VQ for making monster power out of. If you have the resources to chase 1000hp then go the RB26 since there's a wealth of kits and knowledge out there, whereas the guys who build your monster VQ will be trailblazing and your car is the testbed. Actually, in that case you should go an RB30 bottom end with an RB25DET head since killer RB's tend to run single turbo anyway.
But if you're after a value for money, tractable motor that that gives you more than what Nissan offers while being street legal, stick to the VQ35. Unlike buying a CA18DET Silvia (which is a modification dead end POS), where doing an engine swap to an RB25DET / SR20DET is the best course of action, swapping out the VQ35 is not as cut and dried.