A good few years ago I helped someone put a SBC in a viva HC, using original crossmember ( modified on mounts and beefed up ) and no bulkhead or tunnel mods, so it is quite possible to do. I did the wiring which was simple enough, as the SBC had a massive 4 barrel carb on it so no electric box of tricks to wire up.
Incidentallly, the same friend has also shoved an LS1 into a HC ( again I helped with the wiring), this was a fully injected engine management arrangement, the ECU needed sending off to remove the PATS etc but after stripping the loom right back to it's bare bones I got it to run happily with not much problem, It just needed a couple of wires to the ECU for power etc, and it was good to go, not a difficult issue once i'd got a map for the pin outs and knew what went where. Again no bulkhead mods...but its a bloody tight fit.
In the same stable there is a HB with a C20XE living happily in the engine bay, again no bulkhead mods, And many moons ago I put an essex in a HB with no bulkhead mods.
So you see there is no excuse for cutting the shell, it is all do-able with a little thought and preparation.
Although....In my personal opinion, a V8 is too big for a viva, It's maybe ok in a HC, but it's far too big for a HB. A HB needs a well tuned 4pot screamer, it's just that type of car. Have you thought of putting a ford 1600 SE from a focus in there, mated up to a mx-5 box? The Puma also uses a 1700 VVT version which kicks out no end of power and screams its way up to (and over) 7000rpm, plus you can get a focus for pennies, and a puma for around £500, lots more cost effective than a V8.
Or, why not a volvo 5-pot? I've heard tell that a 5-pot gives out pretty much the same off-beat burble as a V8 and can be had for a lot less... but I couldn't say for sure...
Or, why not go electric? There a few companies out there that will supply you with the parts and advice to do it... then you could fit a throttle sensitive sound system that would play the V8 soundtrack you wanna hear