I agree, there isn't much gap between them, there's a reason I don't have but one 60 right now, hard when you have 50's not to just jump to a 70.
Now that you found the saw of your dads is a 254 I think you'll find that makes a great mid sized saw. As Matt was saying they are good saws, and as James said they will take a 346 if both have similar mods or are both stock. I like the 254 as it has a similar feel to a 346, but a little more power, they are actually just over 4hp in independent studies, the specs are a little light on them. That bar on yours is a high quality bar, just ask our Canadian friends
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Bummer about the 180 cover. My neighbor has one if you don't get the cover off I'll get some pictures of his for you.
I think the only cab adjustment on them is the high which will effect the low slightly, but not as much as having a low adjustment.