Bicycle Mechanics - 7 sp HG gear changes?

I have a 7 speed Hyperglide with 30-26-23-20-17-15-13. I'd like to drop the 15, and 13, and put in a 21 and 18.
That would make it 30-26-23-21-20-18-17. How would I go about doing that? I found on nashbar this http://www.nashbar.com/profile.cfm?category=600087&subcategory=60001109&brand=&sku=6213&storetype=&estoreid=&pagename=Shop%20by%20Subcat%3A%20Freewheels
It is a 7sp with 13-32, it comes with a 21 and 18 that I could use as donor gears to my cassette?

Any thoughts?

Harris Cyclery (Sheldon Brown) carries individual cogs as well as some custom cassettes. You could have pretty well any combination you want with the exception that your smallest cog should be one that has the integral spacer and the radial grooves that latch onto the grooves on the locknut.
http://sheldonbrown.com/harris/k7.html

Are we talking about a cassette or a freewheel?

Hyper glide is cassette I would believe...

Hyperglide started as cassettes, but later, for some strange and wonderful reason, Shimano made HG Freewheels:

http://sheldonbrown.com/harris/freewheels.html#7

Hence, my question. The original link (which doesn't really work for me) has Freewheels in it, and the OP mentions a cassette. So we're probably talking about a cassette here but it is ambiguous.

Ok how do I tell by looking at it if I have a cassette or freewheel?

I read the harris cyclery page. I don't understand why so many gear combos have 11,12,13 for their high gears. I almost never usethe 13, 15 on mine. I don't go particularly slow, on flat ground my cruising speeds when alone are often in the 24-26 mph range. My sprockets are 52 and 42. My goal in this gear change is to make the gear ratios close to each other, and then use all 7 gears.
Tonight when riding on flat ground I was cruising 23.5 mph in the 52-20 gear.

So how do you tell a 7 speed cassette from a 7 sp freewheel?

http://sheldonbrown.com/free-k7.html

You'll be hard pressed to find a top cog greater than 13, since the top cog has a built in spacer.
http://sheldonbrown.com/harris/k7.html#7

Well it is a cassette. If I can't dump the 13 then I'll dump the 20 and 15.\
So I'll go from 30,26,23,20,17,15,13
and I'll have 30,26,23,21,19,17,13

How does that sound?

If you do shift to the 13, you're going to have a huge jump, it will feel like hitting a wall. Maybe use the high limit and lock that particular gear out.

On my 7sp HG cassette, the 2nd cog has an indentation for the first cog to sit in, i.e. I can't replace that cog with a flat cog.

Why not use the 42T ring with the smaller cogs.
With a 7 speed, cross chaining really isn't an issue.
In fact, consider a 13-23 using the small ring- IF you don't have hills to contend with.