ever since I plug the new plugs in, it makes that struggling noise above 3500 RPMs.

It's not constant, and it's very slight.

Is it the plugs or something else that is hopefully an easy fix?

Plus my alternator belt squeaks about 3500 RPMs. it's loose. is is normal for it to work loose. Only does this after the engine is hot

 

Is it like a high speed miss? or a bad carburetor bog?
Unfortunately, "Struggling noise" is very vague over internet discussion.

Check them for damage or discoloration, and hopefully you'll find out.

Perhaps the rubber is more pliable when heated and bounces like warm taffy? Could be a lot of things.
Start by using a breaker bar or broom handle to get it tighter. Unless you have a 307 bracket, then loosen up all the bolts on the alternator and tighten the tensioning bolt seated in the cupped flange.

I always have trouble with power steering belts on Olds'
Especially on the 307 junk if you used it on your 350 block.
If your alternator belt runs off of the PS pulley, I'd start by getting them aligned a little better if a simple tightening does not work.

 

if your alternator is cutting out at 3500 it will make the ignition faulter cause it isn't getting enough juice. also if you gapped the plugs wrong or put in the wrong heat range it could make your higher rpm fall off also. things to look for.

 

The plugs are correct for the motor, I had the correct plugs for the 455 in there before. The idle wasn't as smooth, but it was barely noticable that it was rough. I still have the old plugs.

I'm ditching the 100 amp alt, and putting in the new chrome 120 amp I just got on Tuesday

Funny thing. I was stopped at a light with the headlights, radio, wipers, and turnsignal on and I looked at the tach and it was moving like a voltmeter gauge. I lowered the radio and it was idleing that way