IMO:
Firstly 350-400hp from a Holden V8 308/355 combo will NOT be economical, an LS will have better fuel economy in std tune. (Although if you plan on doing lots of cruising picking an appropriate rear ratio it will help mpg, with a T56 you can be a lot more aggressive with the rear ratio <double overdrive boxes>)
T5 4th gear, 1:1, 5th 0.6 "ish" (depends on which box you end up with)
T56 4th gear 1:1, 5th 0.74, 6th 0.5.
LS1 Gen 3 (depending on where it came from Holden or HSV stable) std 235-285kW = 315-382hp, lighter & more thermal efficient hence better mpg. Also lighter than Holden 308. Not sure what the NZ aftermarket is like but an LS1 is currently cheaper than a VK-VL 308/304 5.0/4.9ltr here (Aus).
T56 (std 6sp manual LS option) is on par cost-wise with a Holden T5 world series transmission (VN SS onwards) BUT the T5 won't handle the abuse of a 355 stoker @ 350-400hp. You can buy a T56 bell housings to suit a 308 but again $$$$$$$$.
Depending on if you have a turbo or trimatic bolt pattern on the 308, the bell housing for a T5 comes into play as Holden didn't use a Trimatic block with its T5's option (although they available form the aftermarket suppliers ie Dellow's & Castlemaine Rod Shop etc, don't expect to see them cheap on eBay either).
Clutch pedal & cable: The Commodore V8 clutch pedal is different to the L6 engines so you need to locate a V8 pedal set $$$$$$ (the factory V8 pedal generates more travel, nicer feel <if you use a L6 pedals the clutch pedal sits approx 25-30mm higher & is heavier>) Unless you go the hydraulic route & use the VL style master cylinder then the L6 pedals can be used. If you go LS then a master cylinder will be needed.
Driveshaft: Both options require custom shafts made, V8's d/shafts are different from L6 platform.
The gear lever location will be further back in the cabin with both T5 & T56 options (unless you stumble across a T5 from a VK Group A), not a big issue there are work arounds.
LS platform general requires a later model "K" member for clearance, power-steer can also be problematic depending on the K-member & headers used (a slight mod to the x member or sump maybe required <engineers don't normally like modifications to x members>). Intermediate steering shaft may require changing as well. There is also limited room with extractors, factory powersteer & K member with the Holden V8, but they do fit.
Speedo operation with the LS T56 will require a "black box" & cable $$$$, where the T5 is just a cable (driven gear may require changing) if using STD commodore speedo.
Fuel system will need an upgrade for the LS engine, high pressure pump (8-10mm feed) & return line (6-8mm or you can use factory main fuel line as the return, or go "returnless" system). Fuel tank needs additional fittings or in tank pump fitted. With 350-400hp Holden 308 option an additional fuel pump maybe required, your engine builder should advise you here, most do. HDT came up with a "hot fuel fix" which changed the fuel line routing & added a return line, to over come fuel vapour locks.
Brakes will require an upgrade no matter which V8 you pick, a common upgrade is the VT fronts with VT master cylinder & VS booster. STD V8 rear calipers seem to do the trick ok, invest in some good pads. Will require 15 inch wheels minimum. S/S hose upgrade is not much more as front hoses are custom anyway.
Suspension: A 350-400hp car needs good suspension, don't cut springs nor use cheap shocks. And don't forget the TYRES either as they are the contact between you and the tar-seal.
Diff: LSD minimum, Tru-Trac BW M75 28/31 spline billet axles, it will handle skids. Ratio to suit as mention above. Don't mini or full spool unless it's 100% skid car!!!!!
Electrically: The good old Holden V8 doesn't require any additional wiring, the 6 cylinder wiring can be modified to suit. The LS will need its loom, computer, O2 sensor, additional relays & wiring. There is a possibility to go aftermarket with the ECU ie LINK, Heltech etc. But again additional cost, however more tuneable. If you have AC a larger fan & upgraded condenser is needed over the std L6 option for both engines.
Radiator: A Holden V8 radiator is different to the 6 cylinder ones, LS will require a different radiator again (inlet & outlet different from 308). MAKE SURE YOU HAVE A GOOD RADIATOR or you will be chasing over heating issues!
Fan: Thermo options, plenty of threads available on this site. Stock Holden fan & shroud are ok for 308.
Exhaust: Headers, MIN custom dual 2 1/2, dual hi-flow cats, into 3 inch mufler & 3" tail pipe for either engine option. Everyone will have a comment on this!!!!!!!!!!!!
It would be interesting to see what would be the cheaper option in the end, I'm guessing its 10k (Aus) for an LS1/T56 manual kit plus labour & consumables, 308 with T5 then stroke to 355 there won't be any change from 10k either (depending on the internals $15k AUS on a built 355 isn't out of the question making 350-400hp). Its more like a $20k + investment I'm picking by the time it's on the road, either engine option. Then there's engineers costs.....
I'm not a hater of the old Holden V8's, just the opposite. Thats the combo I have 304 (4.9l), Rochester, T5, M75 3.23:1. I love it.
The sound of a carburetted Holden V8 @ 3000-5000rpm is just magnificent.
Good luck with whichever route you choose.
