Newbie First Build

You should do neither. Buy a used running bike that you can ride and just make upgrades to it as you ride it. I often tell newbies here (but I doubt they listen) there is literally no reason to take a complete bike apart to have a project bike other than cosmetics (eg painting the frame), and cosmetics you do last.

If you really and truly want to build a bike from the frame up - go buy a chopper rolling chassis kit from a reputable company and start there. It will probably take you about 1-2 years of nights and weekends and about $10k all in, and that's the easy route where almost all the parts are supplied to you, nothing is rusty or bent or stripped, and there is a huge knowledge base. Can you do it for cheaper? Sure, slightly, but you really only trade money for knowledge and experience and if you don't have any knowledge or experience you have nothing to trade with.

the truth is the majority of the work of any project bike is research, not spinning wrenches. In order to make something better you must first understand how it works, what others have done in the past to make it better, what are the trade offs, etc....any dipshit can just change parts, but the really great bikes are built with time and patience and intelligence going into every modification.

So yeah, if you want to do this right attend a vintage race and look and see what bikes are most common in the pits. Talk to the racer and see what he/she likes about racing that bike. Then pick the make and model you liked best and search for one in your hometown. Not every bike can be a cafe racer, you need one that has a knowledge base from being raced and aftermarket performance parts support. Don't be one of those guys who thinks any bike can be a racer, because while it maybe marginally technically true for someone with a full shop and decades of fabrication experience, it is not true for you.

You aren't just being optimistic, but wildly optimistic but unrealistic due to your inexperience and general ignorance as to how much work and knowledge is involved.

Case in point: that frame you picked. It has no title. Unless you are intimately familiar with title laws in your state STAY AWAY. You may think you are saving money by starting there but if the frame comes up stolen or your state doesn't have a mechanism for you getting a title, then you flushed every cent you put into it down the drain. You can't just pop down to Home Depot to buy nuts and bolts for this project, motorcycles have specific hardware - where are you going to get the engine mounts for it? The swingarm pivot bolt?

Also I can see it in your posts you think you are going to buy this frame and just start cutting things off of it, but that isn't the way it works sunshine. You would need to build the bike up to complete, figure out what you need to modify, start the modification process, and then start disassembling as necessary to finish modifying. What ever you have in your head from chopper tv shows isn't going to work here.

If you must have a project, then buy a complete project. It's not like cb750s are rare, they are not. A totally complete junker will cost you maybe $1000 at most which is cheap by comparison. If you don't have $1000 to spend on your "project" then maybe you are not in the right time/space in your life for this.

A cb750 is a good bike for what it is. Don't kid yourself, you aren't putting anything in to that frame than another SOHC cb750 engine - the frame itself is garbage, the only good thing about a cb750 is the engine.

I have about 20 years of playing with cb750s since I bought my first one. I have done it every single way and the best way is to start with a running rideable bike. Stop being stupid.