Banging from underneath when shifting

[Budweasel]

Thanks in advance to everyone who can shed some light on this issue.

I recently purchased a 1968 camaro and I am learning about its idiosyncrasies.

When shifting from 2-3 or 3-4 with 3/4 or more throttle I get a loud clunk from what seems like right side of car. Possibly near subframe connector or right leaf spring. Tonight I went into fourth with some more exuberance than before and the clunk came followed by the rear of the car mildly swinging to the left, (I'd guess no more than a foot). Shift happened near 60 mph as I was exiting an onramp to the interstate.

What could be causing this? What should I look for? Broken leaf spring? Loose subframe connector? Axle out of alignment? After losing traction at highway speeds right after the "klunk", I'm having safety concerns.

Background on the car: 489 cubic inch crate motor advertised as 575/575. Transmission is TKO 5 speed. Global West upper and lower control arms, Heidts lowered ball joint, Wilwood brakes, Hotchkis subframe connectors, leaf springs, and front and rear anti sway bars. Caltracs traction bars. Tires are 255 BFG g-Force KDW. I have not identified rear yet, (was told 3.55:1 ratio).