Ok going to check out some simple stuff this week, kickdown linkage, maybe adjust the bands... I changed the filter/gasket last fall, and it was a little low on fluid, but I topped it up and made no difference.

Trans was built by a reputable mopar transmission guy I have known for years, probably around 96. I bought the MP kit, and a 5:1 kd lever.

Problem is this, 1-2 shifts aok, hard like it should be if on the throttle, but 2-3 it will not unless I let off the throttle some, then it is soft. Also when I hit a certain speed (110 on my speedo, but it is wrong...probably 85-90 or so) it slips, like I get nothing.. it does not keep pulling. Never had this before.

74 Roadrunner 400 console shift. Recently had the carb off, this is why I think kickdown adjustment.

Sounds internal.....clutch seals. That would explain the problem getting the trans to shift up to 3rd gear.

In that, do you see it as imminent failure?

I have tons of cores here, and have always wanted to give building one a go...maybe I should get a kit, re-read the manuals and just step into it... In preparation of it failing... Or just pull this one and replace the seals? Is that viable, or is it a case of, its open, do it all?

If it slips as you say, Take a smell of the fluid...should smell burnt. To loose any "drive" at all, there has to be a set of clutches in there that are toast.
That means a reseal and clutches at the least.