What would you want in a custom built tank? A beginner building his first tank need some help 180-200g..

[C_AWOL]

Since it's not entirely clear to me (seems to be leaning more towards a reef based on the frag rack mention though)

For a FOWLR I would go ahead and do something like a 78"L (your mentioned 6'6) x24"W (maxed out width from statement) x30"H since reaching into the tank won't be that frequent in a FOWLR.
Do note that with a 30" high tank you will definitely need thicker than 1/2" to sleep well at night

If this is planned to be a reef tank then 72x24x24 would be just fine at 1/2" and will be easier to light than say a 78". Even more so with euro bracing on the bottom and top (highly recommended regardless of which direction you go)

Assuming you could possibly budge on the width and go with 30" then that does open up the cheap and easy sump options a lot more with widely available standard 100 gallon tanks or even 150 gallon tanks (usually 60x18x24/60x24x24 respectively) while leaving a decent amount of room in the front for miscellaneous items you may stick in there.

For an overflow, the largest external overflow with 3+ holes would be idea if it causes no issues with you getting it down into the basement (very important and something you shouldnt forget!!) but keep in mind that any gate valves you will be using on said overflow configuration needs to be readily accessible for you to make adjustments.
If that's not an option then losing some real estate in the tank for some corner/a centered (on the back) overflow may be more ideal options. Fortunately you can still do herbie/bean style drains on them.

TLDR; 78x24x30 if fowlr, 72x24x24 if reef, external overflow > internal unless space restrictions + euro braced top + euro braced interior for maximum strength

 

[Sassafras]

I would keep the side overflow option under consideration. If you put a 6.6 ft. tank in a 7 ft. space, you would have the +/- 4 in. extra space for the external box and plumbing. You would have the advantage of keeping your 24" tank width and not lose the inside space to a large internal overflow box. Plus, the external box would be easily accessible to accommodate the overflow style of you choice. You could probably find a way to trim out the tank to hide the "ugly" plumbing between the tank and cabinets. Here are a couple shots of a side overflow, not on a peninsula tank. This tank is 30" tall, plumbed as a beananimal and the external Modular Marine box and plumbing adds just under 4" to the overall width.

 

[SteveMM62Reef]

CDA Has a 84” x 24” x 30” 260 Gallon Aquarium. Euro-Braced Top and Bottom. Buy their stand, and get a Fifteen Year Warranty. I have one of their 120 Gallon Aquariums, Euro-Braced on the Top, and their now discontinued plywood stands. The Aquarium and Stand are overbuilt.

 

[ReefEco]

Lots of good suggestions here already. On tank dimensions, I would second the opinion of going wider, 36" or 40" - your options and the natural look (as much as we can for our glass boxes) increase quite a bit. I'm not a tall guy, and though getting to the bottom of my 30" tank is a pain, I also wouldn't trade the look for anything - very dramatic, and the fish seem to swim more naturally with more vertical space, especially if you want any groups of fish like anthias or chromis. It also gives you more light zones for adjusting coral placement, like in a mixed reef. This might not be an option, but go peninsula if you can - it is like having a whole second tank with the second long viewing side. Stay away from rimless for that size if you can, and go with aluminum or steel top and bottom frames. When researching my tank (72"x42"x30") several tank manufacturers said that eurobracing with glass does very little for the structural integrity of the tank, but is merely a splash guard. I got my tank from AGE and they did a powder coated aluminum top and bottom frame with top cross braces. Cross braces are easy to work around for lighting, and are essential if you want peace of mind in a 10+ year tank. I'm in earthquake country, so it was an easy decision here. My build thread has a loot of good videos on creating my system if you want to check it out : ) As for manufacturers - I'd go with a trusted manufacturer over a "friend who build them" - unless I'm misunderstanding his expertise. You are going to put way to much money into this thing to take a risk...