I replaced the starter because the last one started cranking slow and took a while to start. I put an LT1 starter on and its now doing the same thing.

It seems to crank slow in general, but after i run the car for 20 minutes and try to start it again, it cranks very slow for a second then I can hear almost an electrical “buzzing” and the gauges slowly climb up and the car doesnt start.

I’ve wrapped the headers and it didnt seem to do anything. Any ideas what could be going on?

 

Did you check or test the cables and grounds? You might have a cable that isn't doing it's job well. On the job, I've recently had a power cable that had enough problems that the injectors and spark dropped power too. New cables, Problem solved.
My 400 was having a hot start issue last summer, changed the starter to a mini since the old huge dinosaur starter sits right next to the manifold with almost no air gap. Problem was better, still persistent. Checked my cables better and the ground, which looked great, wasn't so great underneath. Cleaned and works perfectly. I should have checked it first but at the end of the day the starter was a bonus upgrade.

 

So should I wire brush the ring terminals to get some corrosion off?

I was thinking about upgrading the Big 3 since I have subs and 2 amps in the back. Maybe throw in a larger starter wire too???

 

I just cleaned the connections off. All 4 wires at the starter. Starts faster when cold now. But didnt solve the issue when hot.

Any other ideas? I saw DEI makes a starter blanket/shield… maybe that would help???

 

A blanket tends to be just a bandaid and not a problem fix. If you're getting that much heat soak from the headers then you may have a lean tune but that's not high on the list. Did you check your grounds? Is the starter well grounded to the block? Did you check for a voltage drop? How much is the starter trying to draw? Did you check the battery? How hot are your headers actually getting the starter? Grab some tools and start testing things. Look for the reason, not for band aids.
Some test you don't know how to do? Look it up.
Don't have the tool? See if a local parts store rents it?
You can do it.

 

Have you load tested the battery when hot?

 

I drove 20 minutes to autozone and got it load tested there. Not sure how accurate their tests are, but its passed