96 years left and I have a substantial point lead. Only significiant opponent left is Ghandi, who is far ahead in the space race. I could probably build about a dozen nukes before he gets close to a space domination victory.

1. Should I just nuke his top 6 cities?

2. If I do, can he still win a Space Race win if he is building the final part in a city I don't nuke?

Any feedback appreciated.

Nuking 6 of his cities will slow him down, but not stop him. If Ghandi is indeed building his last part, even nuking all of his cities won't help you.

However, if there's still hope for you in winning, you definitely need to slow him down. The nukes will help, but may kill your relations with other civs, depending on their relations with Ghandi.
You might try to get some of your friends to declare war against India, and send some troops of your own. Taking a few of his major cities will put a wrench in his spacerace plans.

Can you get a UN win by nuking him and wiping out his population. I think the best way to do it is conventional warfare and try to take Delhi. Nukes will help though. You could nuke his core cities and attack his outer cites and start making your way towards Delhi.

some questions,

A) what difficulty
B) how many parts ahead
C) how many major cities does he have?
D) how far away is india?
E) what techs do you have

but in the end, If it doesn't hurt your own SS production, nuke him. for 2 reasons

1. crippling his production will allow you to get your own ship launched before his

2. If you're going to lose anyway, make him pay

96 years left is still quite a long time to play, and there's no guarantee that nuking him will stop his space-race effort.

As a previous poster suggested, bribing other civs to conventionally attack ghandi is easier.

Also, getting some spies would increase your options, i.e. you'd be better able to work out where to strike and can sabotage as well.

On the other hand, nuking em is good fun.

i never have built a nuke!
must try it one day, just for the fun of the picture ;)

I would go for the bribery thing and capture one or more of his cities.
Better to capture his big production cities than to nuke them.

Edit: this doesn't prevent you from building the nukes, after all, nukes are efficient weapons, aren't they?

Spies are the way to go. Send your spies to his cities where he's building SS components and sabotage them. When the spies die, keep building new ones to sabotage his production. Actually, if you let him get within maybe two turns of completing each piece and THEN sabotage, you'll hurt him even more. The same applies for wonder races.

do all the above

Hopefully Nuking him (and the ensuing war) will make him switch to a war footing to go after you as opposed to continuin the space race. After the nukes go off THEN send in the spies to keep tabs on whether or not this has worked. If it doesn't then you're totally screwed. And you're screwed even more if he has the SDI (way overpowered).

Nuke him if you can easily win the war(s) that will follow. If you can't, start thinking about Plan B.