Only gear it slips in is first, and only for the first mile or two, then the slip goes away, never slips from 1-2 or 2-3.
Planning on
-running seafoam through the trans
-driving it for a few miles
-dropping pan
-add drain plug
-drive it a few miles
-drain fluid
-refill
-drive a few miles
-drain fluid
-refill
-drive a few miles
-drain fluid
-refill and add a qt of Lucas

Any ideas as to what my issue is, other than What i am expecting(a rebuild)

 

Make sure the trans pan has a magnet and are you planning on replacing the modulator? That might be the problem? If you pull the vacuum line off the end of the modulator and it has trans fluid in it, its bad.

Its a simple match the color band and thread it out and the new in. Also ck all the vacuum lines for dry rotting or cracking.

 

Don't think your are going to do anything other than spend time and money with your plan.
When you say slip, do you mean a delay in engagement of forward gears? If so, and it only happens when cool (as in first start of the day) it is the forward clutch seal. No other fix other than replacement. That means overhaul.
If it slips every time, try starting out in manual low. If that clears the problem, the oneway clutch is bad. Again trans needs to come apart.
Transmission components active in 1st gear (auto) are forward clutch and overrunning clutch. In manual low the low/reverse clutch is applied to assist the one way clutch.
Hope this assists you

 

X2 on the forward clutch seal. This is how it starts and, over time, will get progressively worse.
This is a common C6 problem.

 

Wonder .... how many miles has she seen?

I think the worst part is the R&R of the transmission on your truck (if it's 2wd, being a F100 ... I'm just guessing) with no transfer case. C-6s around here don't cost a fortune to have rebuilt. In fact, you might even prefer to let the trans shop do the R&R too.