I'm guessing from the pics that you're still fairly young. Dude, you're going to be so happy. What you're dealing with right now is in some ways worse than being flat out bald.

Either of the options you mentioned will work. But you're going to need a template whether you're cutting a stock system on your own or ordering a custom partial. There's a "how to" tutorial on the TopLace website that will show you how to make a template. Just pop the thing on your head and draw the hairline that you want. Draw it and you WILL have it. And if you are young, as I've speculated, don't be afraid to lower that hairline in the middle by an inch or so.

I will throw one last thing out: you're thinning all over, so you might want to think about shaving the entire top and going with a conventional topper. I guarantee it's easier than what you're planning to do. But, in the end, it's whatever makes you comfortable. You can get outstanding results either way.

Best of luck. You'll be glad you did this.

Edit: I just re-read your post and it sounds like you're wanting to integrate your existing forelock with a system that begins behind your existing hairline in the center. Dude, it can be done, but it's more difficult. I would urge you to either get a partial that runs in front of your existing hairline (that's what I was referring to when I suggested you lower your hairline a little) and then curves around to fill the receding areas in the temples, OR just shave off the forelock all the way back to an area that is still pretty thick and work with a partial going forward. It's just an opinion, but I see no point in trying to integrate seriously thinning hair. It's going to be gone eventually, so you might as well shave the crappy stuff off and put some good stuff up there right now. Again, just an opinion. And I'd seriously consider shaving even more, if I were you, and going with a conventional topper. An added advantage to that is you wouldn't have to worry about matching density with your already thinning top hair if you went that route. Your side hair looks great, so I think the blending process would be pretty simple for you.