318 poly wide block

Please don't call these a "wide block." If you stripped one of these down to the block, and set it next to an LA, you would have to take a "second look" to see the difference. "Poly?" Just fine

These are properly called an "A" block (vs B block for the big block and RB for "raised block"). The "LA" gets it's name because it's a "light" A block

So far as clutches, Any Borg and Beck, whether Poncho, Chev, Dodge, or "something else" has the same flywheel pattern in a given size/ configuration.

The clutch disc must fit the transmission spline, AKA Ford gearbox in a Bronco mated to Chev SB, use appropriate Ford clutch disk

In something that big, "heavy truck" I doubt it's a 23 spline transmission. If you cannot find a listing you are in the wrong parts store or talking to the wrong parts "person." If you are in "any kind" of city look for a "clutch and brake" rebuilder

You could also put a diaphragm clutch in there, but these are generally not thought of as "heavy truck" pressure plates.