I've rebuilt a daily driver /6 but now I want to build something with a little more kick.
What would be a good donor block? I'm leaning towards a / from an early 80s pickup simply because of hydraulic lifters. Is there one year that is more preferrable than others?
Is there a big difference between the quality of a cast or forged crank?The way I understand it is they are not interchangeable.
Is there a piston style that would help put a few more ponies under the hood?
Is there anything I should definitely not do?

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70 Valiant Super Six

I guess some people prefer the 170's cause they have a shorter stroke and can rev higher.

All '81-up slant-6s had hydraulic lifters. Hydraulic lifters are nice from the standpoint of never having to adjust the valves, but the stock hydraulic cam is very much on the "tame" side, and the selection of aftermarket hydraulic cams is even narrower than the selection of aftermarket solid ones.

All '76½-up slant-6s had cast cranks. Neither the cast crank nor the forged one is known for weakness or tendency to break in daily driver service, but the forged crank is better/stronger and the bearings are larger.

The 170 can rev higher as JD says, but has significantly less torque and is not nearly as nice in daily-driver service. Being able to rev higher is an advantage in some kinds of racing.

As far as pistons and such, people have used lots of different kinds of pistons successfully. Which one to pick depends on what you decide to do with your rebuild (stock or near-stock bore...great big overbore...something in between...)

The thing definitely not to do is to go overboard and put in too much camshaft, too much carburetor and/or too much exhaust.