delta 3200 is actually a low contrast film designed for pushing with a speed increasing developer resulting in normal contrast.
If you are using Microphen stock as per ilfords recommended dilutions with microphen, then shoot at 3200 and you will get a normal contrast index negative with ample shadow detail (an 8 stop range). However, D3200 and Microphen combination will be quite grainy which may or may not be what you want.
But you say you are using D76 and that is at least a stop slower than microphen with D3200.
Ilford work out ISO using ID11 (same as D76 near enough) so that speed of 1000 or 1250 will be about right with D76.
But if you must have the speed of 3200 or 6400 then microphen will be your best option, optimally at EI 3200.
Thanks, but unless my local photo supply store carries Microphen, I'll probably just stick with the D76 for now. I'd hate to risk messing up with an unfamiliar developer on something more important than a test shoot!