skids

327-350 in my 64. Had it on the chassis dyno for a day, and the car runs great. However, between gears, 4500rpm and above shifts, there's a crackle,pop out the exhaust. Not a backfire. Unburnt gas? Can't really remember, but I don't recall having this issue in warm/hot summer months. Tried ported and full for the vac. adv., didn't seem to help. Only change I made after the dyno, was replaced open carb spacer with four hole. Motor liked it. Then rejetted/ squirter change, Holley, according to what my dyno suggested. Any ideas?

L78racer

is the exhaust crossover in the intake blocked? is so, add an 'H' pipe or 'X' pipe external crossover. it will sound much better.

Don Keefhardt

Leaky exhaust manifold gasket, cracked header, badly sealed junction header-to-downpipe, etc.

A popping or crackling sound on overrun is a pretty good symptom of an air leak someplace where the gasses are screamingly hot and moving at big velocities, IMO. The exact mechanism of the noise generation is kinda fuzzy, but IIRC it has something to do with unburnt fuel (carbs are wonderful, aren't they ?) and air/oxygen getting sucked into the flow (Bernoulli is dead, but his "principle" lives on) ignites little "microbursts" of fuel.

Or something like that.

Lived with it on an old Norton Commando. And an old Ducati Darmah. And on various race cars.

skids

Well the exhaust leak might be the culprit. I have a Performer RPM intake, forgot if it has a crossover or not, or if it does, if it's blocked. Probably not. I have Hooker stainless sidemounts, new, and I did recently replace the sidetubes. Maybe one or both have a slight leak at the collecter connection.