Realistic estimate Please

OK so you could probably have the slant out and the V8 bolted down in about 4 hours.

Each front mount is (1) nut.
Rear crossmember is (4) bolts plus (2) nuts
Spedometer cable.
Shift linkage
Fuel line.
Wiring harness 1 nut on alternator, temp & oil senders, 1 nut on coil, 2 on starter.
Negative battery cable
Throttle linkage.
Radiator, fan and hoses.
Exhaust flange
And thats about it.

Reverse the above and you're in business.
Need V8 throttle linkage.
Need V8 radiator and hoses
Need V8 wiring harness (or modify existing)
Need totally different exhaust system.

So, if you have all the pieces handy, like perhaps a V8 parts car in the driveway to rob parts off of, I'd say a weekend.

If you don't have all the "little stuff" all bets are off. If time is an issue, like you need to drive it to work the following Monday, make sure you have as many parts lined up beforehand.