I figure it's b/c of your cam, M-21, tires and rearend gears not being better matched.
My 406 SB with a 292/230, 109/107, .480" lift hy-cam, M-20, 3.31's and 26.5" tall, old L-60's was still on the cam at darn near 50 mph in 4th!!
And around town, cruising, my wife use to really get upset when it started to "hunting" in low and second, snapping the driveshaft!
BUT once "up on the pipe" as the two-stroke guys say, it ran like "Jack, the Bear"!!
I was glad it was stick b/c I could just immagine how it woulda bucked with an auto trans..