What should I set the timing at?

Advance it till it starts to ping, then back it down 2 degrees or so. Depending on the condition of your motor and compression ratio, etc..., you should be able to get almost 40 degrees total advance without it pinging. I would start at 10 degrees initial advance and work from there. If you dare jump into the centrifugal advance adjustment and/or vacuum advance adjustment be patient, and adjust one at a time.

The timing changes at a certain rpm right?
How do you adjust that?

On a car with vacuum advance, the timing will advance due to two factors: 1.) the vacuum created by the engine and 2.) the comination of weights and springs that provide a mechanical advance.

Always check base timing with the vacuum advance disconnected and plugged.

Usually total mechanical advance is reached at 2500 rpm or so. This will vary due to numerous combinations of weights and springs, along with the age of the springs.

Base timing is not as important as total advance. I like 36 degrees total advance. With my setup, I think my base timing is 8 degrees.

read this thoroughly!

http://64.90.9.168/cranecams/pdf/272e.pdf

Start at around 10 degrees BTDC and work your way forward until you ping and back off a little. I would guess you will end up with around 15 degrees BTDC. Note, details like compression ratio, condition, grade of gas, elevation (yes, feet above sea level) plus many others are all factors that affect optimal performance. But between 10 and 15 is usually a good place. This is one of the advantages of the newer EFI systems, it does some self-tuning.

The timing changes at a certain rpm right?
Actually the timing changes up until a certain point. On a stock distributor, you may never see the total advance the distributor is capable of, because it comes in so slow and way late.

How do you adjust that?
The amount of total advance is changed by either changing the initial timing, changing the weights in the distributor, or both. The rate at which it advances is controlled by changing the springs on the weights to lighter/heavier springs.

I have the specs for fords distrib set up, used in 289/C4 applications. Interesting to note max vacuum advance is about 9 degrees, max centrig advance is about 13 degrees with 10 BTDC starting that gives 32 total. This was plotted for me by Kogers distributer dynamics whom I highly recommend. Send him yours, he puts a pertronix in and recurves to ford standard. Also interesting was that he has tested many off the shelf distributers. Only about 1 in 10 is correct! Even the ford units! He has done two of my cars.