I guess it's splitting hairs, but does one actually put one of these V8's "in" a small Datsun, or does one build a new car using the Datsun Bodyshell & the V8 as some of the component parts.

None of the original driveline would be retained, so a new rear axle assembly would be needed & it would probably need narrowing to fit, next, the front end will require serious upgrading, & possibly total replacement, depending on the choice of engine & it,s location in the chassis. The structure [chassis etc.] would need major re-inforcment, & the decorative part of the body [the skin] looks like it will be compromised in order to get some sort of weight balance, particularly if running tall induction.
In order to benefit from the big horsepressure, tubbing looks to be one option to allow large tyres & this could also mean radiusing the rear wheel arches in some instances.

For competition use, i believe that some major re- engineering of the rear suspension would be in order, particularly rear coil sprung models. For street use, the chances of using original leaf springs looks pretty slim too.

The firewall mods would likely require a revised heater arangement, if staying street legal, along with whatever was needed to accomodate the new trans tunnel & i guess a full rollcage would be one way to stiffen up what really is a fairly flimsy body.
Lets face it, as the factory saw it, they really needed only to be strong enough to hold up under relatively normal driving loads with a 1200 engine.

After doing all that, has one really just put a V8 "in" a 1200? Compared to say, an A15, or CA18DE conversion.