If you are not out to spend a bunch of dough...
Why not do a 393?
The price difference between 393 and 408 is huge.
The 408 uses many 'custom' parts, while a 393 only requires one custom part.
The rest of the combo is 'off the shelf' parts, so to speak.
The first thing you need to do is find a machinist that you can trust.
He will check the deck surface and tell you if it needs milled or not.
He will also, with your direction, check all the other machined surfaces in your block and let you know what needs attention and what is in acceptable condition.
You can also have it checked for cracks and such. This is considered a good idea because you don't want to build this thing and find out later that it was a 'grenade' waiting to happen.
Things that may need done, or may be a good idea depending on what your machinist finds:
Decking the block for trueness
Line boring
Boring (.030" over bore is safe, but .040" and .060" can be done)
Cam bearings (consider this a must)
Boiling the block and brushing the galleys (consider this a must)
Freeze plugs (a must if you get the block boiled)
Make sure he puts all the galley plugs back too! (this is why you want a good machinist, a hack can mess up alot of important things and make you block worthless in about 3.4 seconds)
You, or the machine shop should also chase ALL threaded passages with a tap.
A really good machine shop will bore your block while it is heated to operating temp, with plates installed and torqued to simulate having heads installed.
A top notch shop will also do all machining in relation to the main webs after line boring, so everying is perfectly square. Many times the blocks come from the factory out of square. They were made fast, not precise. So if you bore and machine by using the squareness of the block, you are just reinforcing the out of square condition.
Dave