I pulled my 258 and I'm doing a 4.0 swap...well it's been bored and I'm using my Eagle's stock Crank and rods, so it's going up to 4.7L.
I am reading Joshpit's write up, and while mine takes many deviations, it's been quite helpful. I do not have the engine assembled at the moment, and this could be answered by doing that.
Joshpit's write up says you need to use a 32RH OR a 42RE flexplate. Why can't I use my Eagle's stock flexplate off of the 258? There would be many advantages to using it, especially since it's in good shape for being 40 years old. I'm almost 40 myself, and I am in worse shape than that flex plate. I am assuming he recommended the 32RH and 42RE flex plates because he was still using the 4.0 Crank, and I will not be,
The flex plate has to be changed if you are running 4.0 fuel injection. The flex plate has the tone ring for the crank position sensor.
Another option for the crank pickup is from Hescos if you plan on using the fuel injection
https://hesco.us/products/30791/40l-conversion-parts
You may want to read through Tougeagle's thread also:
https://forums.amceaglesden.com/index.php?topic=21335.0