Which 250 six pan and timing cover to use

[bubbasimms]

I have 2 250 six engines, one is at the machine shop. I was picking and choosing the best tins to clean and paint and noticed a difference in the pans and timing covers, specifically, one set bolts together at this junction and one set doesn't. I made a post over in the engine section and learned that there were early and late 250's. This brings me to a couple more questions....which is early and which is the later? Which set should I use? I was also told that the rear seals are different, so how can I determine which gasket set I need? Is it just the seal or difference in the rear of the crank? When was the change? The casting number shows it as 64-76 230 and 250 or something like that. I don't want to put together a leaker.