Need Advice Tunning My Truck In

[Hair]

71 GMC 250 Six with powerglide

Transmission would not shift into high gear. Pulled it out and took to transmission shop and had it rebuilt. In the mean time I also put an HEI unit in instead of points. The transmission shifts well now, and the engine runs fine, but here's my problem:

When it's in Drive or Reverse and sitting still with the brakes applied it runs very rough, not a miss, but like its lugging or trying to pull, it's feels like it needs a stall converter (but it don't, its all stock). If I idle it up high enough to run smoother I can REALLY feel it pulling then, like I'm trying to brake torque it and then it's "screaming" in Park.

I'm wondering about two things:
1.) I put Type "F" transmission fluid in it, hoping for a firmer shift.
Do you think that this could be my problem? Maybe someone has experienced this before? Should I drain/suck it ALL out as best I can and put Dexron III back in?

2.) I did not install a new torque converter.
Do you think that maybe the torque conveter could be bad? What does a bad torque converter in a power glide act like... never had a bad one.

Maybe something else?? Any advice appreciated. Thx