Rebuild or leave trans?

I am adding some power to my motor and adding some 3.91 gears to the rear of my 73 dart, and just started thinking about the transmission. The trans is a 999 with a low first and second gears. My old 360 had mid to high 300s for power, I really am just putting in a bigger cam (hydo flat tappet to solid roller) so It will probably have a tad over 400HP. Last time when I put it in the only thing that I changed with the trans was add a transgo tf2 shift kit, otherwise the trans was stock as far as I know.

The transmission shifted really nice and firm, and it never slipped or anything, seemed to run just like it should and easily handled the power (no racing but some spirited street driving). Because it ran good before and I am not adding a ton more power should I just leave it as is and run it again? Or should I rebuild it first? from what I've been reading they are pretty easy transmission to rebuild, but I've never done it before.