They can be spendy is one reason I can think of. I was trying to get either a corviar or vega box for my project car & they are in high demand. hence the places that sell them new tend to chqarge what the market will bear,...
Flaming River has a great tech dept., so if you have particular questions droppin them an Email should get you solid answers, and price estimates.

What I did end up using on my car was a late 60's Ford Mustang box. Has fewer turns than a stock corvair box and is much easier to find.

1956 GMC 370 dump " 'Tater "
1970 VW Volksrod "the Black Bomber"
2007 Chevy Avalanche
2020 Jeep Cherokee Trailhawk,
2005 Harley Davidson FLHTCUI

I dig all cars, old and new, whether they were hammered out of American iron, German steel, or Japanese tin cans. Being unable to appreciate them all is missing out on a world of great things.
But thats just MY opinion! :P