the viscous coupling in the fan locks up and not only makes it sound like an areoplane as you rev it up (the voscous bit locks it up as it gets hotter and unlcoks as it gets cooler, the same sort of way as say an lsd locks up the faster on side starts to spin) it will also use more fuel

the only real way to test if it's failed to lock up is to see if you can jam a wooden pole in the fan to stop it, but it's a risky thing to attempt if you've not see it done before so i'd personally not go that route

the V6 fans were bigger than the pinto fans so you can't just swap them over

as for the twin fans, the dohc ones are a bit hit and miss when it comes to fitting them, but you can always try and move the rad forward in the chassis holes underneath, fit them to the air con condesor holes and see if there is enough space then