Cronatron 211 is a cast iron welding rod. It works very good. It is a stick welding rod, but i have cleaned the flux off of it and tig welded with it. You don't have to heat up the material to be welded but it needs to be clean from paint oil etc. Thats a pretty bad crack and its up to you but I have used it on a riser and it worked fine.
Mike

The comparison of the tire to the engine is pretty poor. If you get a flat tire you can walk home or to get it repaired. Where are you going to walk to after your boat sinks???

Fellas,
Thanks for the wealth of info. and entertainment! LMAO!

Our welder here in Cody Wyoming said he could weld it, but it might hold for five minutes or five years. He's welded cracked tractor blocks that have lasted for 15 years, and others who have split in 5 minutes. Apparently it's a 50/50 shot welding cast. So we tore it apart, and it was a good thing, she's cracked inside as well. We're taking all the good internals to a reputable engine builder who'll prep a new 305 block and build us another short block using what we have left. We're having the heads mag'd as well. They look okay, but one never knows. The OMC iron exhaust manifolds are cracked as well, so I'll start the search for another set.

Here's a pic of the internal crack.

Thanks again to all of you for your insight, my buddy, a.k.a. father couldn't believe the amount of feedback we received on this. He's old school in terms of the internet and is still in dismay.

No real surprise...
 

I had the same thing happen to a 4.3 V6 (3/4 of a small block), and it looked almost exactly the same as yours - not too bad on the outside, but !!!!! on the inside. It was cheaper for me to buy a complete OMC short block assembly than to buy a bare block and have someone reassemble my internals into it. If you're doing some of the work yourself, that will help, but you may want to have a hard look at going with a 350 replacement (marine) short block. The reman shops have them pretty cheap. Boat gets faster, too. :D
While you're at it, you may find that the performance exhaust manifolds (IMCO, Kodiak, etc.) may actually be cheaper than new OMC ones. Boat gets even faster. :D :D