How much HP do I need for easy passing at 60mph?

[Beatcoaster]

I am looking at swapping in a new engine or rebuilding the tired 307 in my 69 long bed. I used to have a Subaru STi that would run 13.0 in the 1/4 all day long, not fast by any means, but great fun on the street. I absolutely loved the passing power it had, and it was put to good use lots as we have some property on the other side of the mountains. We make 2-3 trips a month, and the roads are single lane highways nearly 3 hours of the trip. With that said, I want my truck to have the same passing abilities as there are many occasions where it is needed. I can't imagine any rebuilt 307 letting me pass comfortably so I think a new 350/355/383 in going to be in order.

Now back onto the question. My truck has 3.08 rear gears, and a TH350 tranny. I debate on the "cheap" GM crate 350 with 250hp/350ft-lb's but just think it can't/won't breath enough to make passing quickly a reality. Do you folks think the Blueprint 355 w/310hp and 360ft/lbs would be enough or do I need to save a bit more and step it up to 375hp/400ft-lb 355? Or even blow that out of the water with a 383 pushing 400hp/450ft-lbs? I just don't know what to do at this point, and don't have enough seat time with this old ride to know what I need for power. Any ideas or thoughts from your experiences?

Thanks