Both your 4276a and 4263b can measure ESR. So you may take the Henry Parsons Hall way, which he did at general radio with the digibridges:
First he calibrates the measurement-frequency deviation.
Secound he uses calibrated resistors and no capacitors for calibration.
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For your case: Check the oscillator frequency for drift.
Then take your calibration resistors and measure them with your 34461a. Now measure and tweak your 100pF and 1000pF calibation capacitors until your calibrations resistors measure the same values on the 4276a as on the 34461a.
Build from relative new (post year 2000) high voltage foil or Y2 capacitors - they have very low D, but D is never nagative.
Check with your 4263b and DER-EE 5000 - both with short and open calibration.
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By the way: my 4274a/4275a did not drift out of spec in the mid ranges, for me f > 1 MHz ranges are hard to check . Maybe the 4276a behaves the same, if you didn't tweek it. My 4263b drifted less then 1%, but more then the 4274a, if i remember correct. But all are not in the class of the genrad 1689 which has the most simple design and calibration procedure and like the 4274a no special parts.
(the 4274a/4275a are a repair nightmare compared to the digibridges, but have the better user interface).
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