Being that the AMC gremlin is much lighter, and a small tire no prep, the gearset may last a while but it’s a toss up. I’ve heard of them lasting in lighter cars. The 1.80 straight cuts would definitely be the way to go for longevity. The aftermarket case is definitely a nice touch to not running a blanket or shield. I’m sure their pro glide with ultra case and bell is upwards of $5k, if not more. Plus converter. And I agree, for what you’re getting, the ATI is not a terrible price.
If you end up buying this one from Bill, you could always put the 1.80 set in it.
The stock case shouldn’t be an issue for what you’re doing in my opinion. It’s more convenience for not wanting to run a shield or blanket where the ultra case comes in. That and when you start cranking up the pressure from power adders, the stock case comes into jeopardy. I know my car is heavier than 2500, with 850+ HP, and I wouldn’t have an issue putting a stock case in my car. Just my opinion.
Talk with your trans builder. He may have different experiences putting the 1.76 behind your power level and see what he says.