HEI Distributor for 258

Oliie... what is the 258 going in? Are you swapping it into the Rambler? 

The 250 dizzy swap... Isn't that just an 6 cylinder version of an HEI? I would say they are more reliable than the Motorcraft but the Motorcraft TFI upgrade stuff and the Ford 300 Big Cap is totally use-able still. 

I think the same Isky 256 cam in the 258 will be fine. It will likely shift the RPM band up a little bit and provide some extra torque but for the most part it should behave the same as long as the the engine is in a similar sized vehicle.  

I used a gear from an AMC V8 from 81 or 82......... the gear fits the GM HEI shaft and is a perfect match for the 258 cam gear.  The early AMC V8 distributor gears are wrong size for the HEI. The later V8 used electronic ignition and a smaller shaft, matching the GM I6 HEI
You will find those gears being sold to JEEP nuts who also want to convert to HEI - Jeep shops, eBay, whatever, they are technically the later V8 gears, not the GM points distributor gears. 
You MAY need to grind the shaft flats up the shaft a bit so it won't bottom out in your oil pump and stress the pump.
What I mean is that you should install it with NO gasket and make sure the shaft has end play.

If it's snug with no end play, pull it back out, use a small angle head grinder and grind the shaft flat part up a ways making the flat part longer up the shaft. 

Try it again.

What happens is that the flat part doesn't always extend up high enough and when you install the distributor when things get good and hot, it ends up pushing down hard on the oil pump because the round part of the shaft hits the oil pump shaft.
Once you make sure there's a bit of play, put the gasket back on and that guarantees play.

I think I have a post somewhere about putting HEI in the AMC 6s........ what I use, how I had to modify the HEI shaft a bit so it would fit down into the pump properly, etc.

I have a comp cam in my 4.0 and if building a 258 - I'd use the same 4x4 cam in it, too. Love it. It smacks the valves open and closed faster so it's not a quiet cam, and I used H-S roller rockers I bought from Mike Luke in oHIo, love those, too.
I've got a ton of torque and can easily accelerate going UP some of our toughest hills around here without downshifting (and no, Iowa is NOT flat - especially where I live!)

The gear off a 232 point distributor will work as well. It shares the 0.490" shaft diameter. Gear location measured from the block mounting flange was within 0.020".

http://www.amceaglesden.com/images/Downloads/MIU.pdf