Your amplifier probably can't go any lower than 4 ohms - so of course 2 8 ohm cabs equals 4 ohms. I would say wire both your cabs with 16 ohm speakers in series/parallel for a total of 16 ohms per cab so that when you combine the 2 cabs your total ohmage is 8 ohms which is safe for just about any setup. Since you are using a 100 watt head, I suggest you use 25 watt or more speakers, so the grand total will be equal to or greater than 100 watts.

As for the outside, I recomend getting a router and even if only very slightly rounding all corners and edges where your fabric will cover, so that when you pull it tight it doesn't tear anywhere. I use this stuff by 3M for gluing fabrics - I have done cabs with tolex, vinyl, patterned fabric - I have done it more than once as you can see.

Lay the cab on it's TOP, so that you begin working at the bottom where the wheels go. Spray some glue, spread it around, and stick your fabric to it only covering the one face [not the front or back]. Staple the fabric to the cab where the wheels will go later, so that you remove the staple when the wheel is installed [after the glue is dry]. Roll the cab onto it's side, spray the glue, pull it tight etc. and staple where your corner covers will go. Smooth out the bubbles as you do each side if you are working with an air tight fabric like vinyl or tolex. The main deal here is to work in a clean area because if anything gets caught between the fabric and the wood, it'll show. The problem is everything sticks to glue, so you have to be damn careful. When you get back to where you started on the bottom, overlap the fabric - and using a NEW BLADE ON AN EXACTO KNIFE AND A STRAIGHTEDGE, cut through BOTH layers of fabric. It is important that your blade is RAZOR sharp, and do it in ONE stroke - pull out the scrap from underneath the second layer and fit your fabric together, it will line up perfectly. This is where I let it dry.

Now when you get back to it, pull the fabric back and cut out from each corner, spray with glue, fold the fabric in, and staple the hell out of it. All your seams should fall into the corners and be near-invisible if not completely so. The same method applios to both the front and back of the cabinet. Then, where your wheels go, if they just mount on the outside of the cab, then install them over top of the fabric. If they are the kind that gets a hole drilled then the base installed, and then the wheel screws into that, just measure and cut your holes where you need them., then install your front and back panels after you have applied your fabrics to them. Or install a metal grille on the front if thats what you want. Add your coner pieces and whala!

Where large holes will be [for example if you have recessed handles or a large jack plate on the back] cut an X in the fabric, corner to corner, add glue, and fold them inwards, then install your handles/jack plate/ashtray in the hole.

Eds you seem to be know what you are talking about so I got a quick question for you, I have this cab

http://www.musiciansfriend.com/product/Behringer-Ultrastack-BG412S-4x12-Stereo-Cab-with-Jensen-Speakers?sku=480672

and this head

http://www.musiciansfriend.com/product/Marshall-MG100HDFX-Head-?sku=483103

if I crank out all the volume I can from that head is there any danger of blowing speakers or such on that cab or since its a 100w head and 400w cab will it be perfectly fine, i've never tried for fear of damaging my equipment

In theory, yes. In practice, probably.

100 watts is a pretty big generelization - there are dips and spikes to every power suuply and a surge with your amp on ten will probbly do some damage, though you may not notice it right away. Also, clipping your power amp's input [an amp head is divided into two parts, pre and power amp - the preamp shapes the tone while the poewr amp amplifies it] so by putting too hot a signal through your amp via incorrect settings on your guitar pedals, plugging stuff into your amp that you shouldn't [like a cd player into your input jack, especially using something like a headphone output] or changing the gain level in your effects loop, you can clip your power amp which will cause your voice coil to heat up and burn out fast. This can even be done whjile using low to moderate volume setti9ngs, don't think your amp has to be loud to blow [incorrect gain staging is the number one reason for idiots to blow PA tweeters!!!]. So yeah, doing that can bugger stuff royally. But it takes a special kind of fool to do damage like that.

Also impedance mismatching can damage your amplifier, though a solid state amp is actually way more tolerant to imp[edance mismatches than a tube amp, because a tube amp "expects" to see some speakers at the other end, and solid state amps kinda just dont care one way or another.

So basically what I'm saying is match impedances, dont overload your amp, and if you find somewhere to crank one up to ten, I'd be amazed. Two years from now, if you're a gigging musician chances are good you'll wonder why you never took this advice: Buy yourself a closed back 2x12 like the Marshall 1936... and maybe you'll have enough room in your car for some hot groupies and a back still strong enough to handle the action associated with that.

chances are unlikely you will blow out the speakers with that head. as eds stated, solid states heads aren't all that particluar to the speakers used. the REAL issue with solid state amps are that they really break up TOO much at peak volumes creating a rather nasy sounding crackly buzzy distortion. solid state amps are best run at low to medioum volumes for optimum performance. i don't imagine you'll have much problems with that setup, but i doubt you'll really like the sound all the way cranked up......