I have found backfire thru carb was caused 80% of the time by timing.
10% of the time by valve issues like sticking intake valve or burn ones.
the last 10% I reserve for the unknown havn't see that yet disclaimer.
Your choke issue sounds very much like shes flooding.
It is possible your getting to much squirt and flooding out rather than leaning on quick throttle bump. <like hitting it with a can of carb cleaner it will bog and recover>
Check the float fuel stops for leak by.
Check the timing , use the back yard method of mark current location and advance it a bit. see if this has an effect. You mark it so you know where you started and can easily return it to that position.
Check the valve train
To get fire in the carb you have to have an intake valve open when there is flame. This is usually timing but, can be fire still in the cylinder as the valve opens or the mixture is hitting a hot cylinder <preignition>.
725 seems like allot of cfm for a 383, but the above link shows a 720 is needed at 6500rpm.
what vacuum is it pulling? Compression ratio? Cam Specs?