Larry when I had a warmed over 440, I ran the stock 18 spline 4 speed with no problems and always power shifted it at 6500. BUT when I built a big ol Hemi that had a lot more power and a lot more RPM, I had to go to a crash box/ slick shifted tranny. I left the syncro in first gear and only modified 2nd, 3rd and 4th. By leaving all the teeth on first, I think it was better on the trans on launch and this way I could still get it into first without coming to a complete stop on the street or return road. I didn't run a slipper type of clutch. Never broker a trans, but went thru them and serviced them all the time to make sure no problems.
I tried the tranny fluid but didn't like it. Didn't seem to shift as well or as smoothly. Went to a mix of straight 30 weight oil and some B&G gear additive. Some really slick stuff. I ran the stock 4 speed springs with 30X10.50X15 slicks. The rear was a Dana 60 with the stock posi unit. Had to replace the clutches in it once. Ran the stock 3.55 with the 440 then when I put in the Hemi, I went to a 4.56 and a spool and of course bigger slicks 30X12.50X15. Still ran the Same trans forever with just regular mainenance.
I also tried one of the bronze 6 puck disc. I thought it was pretty nice and a lot lighter thant the full face clutch disc.