Just installed a denso 1 wire alternator. I believe the output is about 65 amps. I believe that I need a breaker btwn the alt and the battery. Can't find a stud style breaker btwn 75-100 A. I don't have enough space to put one of those bulky breakers, not much free space in the engine bay. Anyone have a solution?

I use a 50amp fuse

Why do you think you need anything as protection between the alternator and battery?
I don't use and I don't think any protection is needed as long as you protect the wire and connections

Use a fuse if it's for some kind of protection. Use a 200amp cole hersee can solenoid if you're wanting to interrupt the output for making passes.

If the motor is running and you have a fault in the power cable, then you will feed the fault from both the battery and the alternator. I always fuse at the alternator and at the battery. I don't use breakers, just fuses. Think I have a 120 amp at the alternator and 300 amp fuses on the battery.

Commonly used in aircraft:

https://www.aircraftspruce.com/catalog/elpages/70amp.php

I used this, and it works fine, but the circuit breaker looks like a much better setup.
Napa inline fuse

Never used one, but if I did, it would be at the battery, not the alternator. MIDI fuses are available to 200A I think.

This is the 300 amp I use at the battery.

https://www.waytekwire.com/item/46279/EATON-s-Bussmann-Series-AMG-300-High-Amp-Fuse-/

I used this stuff on my boat.. I just mounted it on a bracket.. its still
on there.. I used 2 of them and its on my race car at both ends

I remember that style breaker on some military aircraft from the 70's. Must be a good design if they are still in use. Thanks

I don't remember my 1960 Piper 250 Comanche having one of those, it had a generator originally and got converter to 260 HP motor from the firewall forward before I bought it though I do remember having a on and off switch mounted under the instrument panel for the alternator, maybe it had one on that also
Probably not a bad idea though when your at 12,500 ft. in the clouds and something starts to smell like burning wires

A really high amp fuse is not going to protect anything in the car unless it's located at the battery and the main cable gets shorted directly to ground. Everything in the car will go up in smoke before it breaks a 300A fuse.