[aerohead]

Since you mentioned this, I re-visited the research done on the Ahmed body.
Some data points:
* when the rear is modified, this new 'aft-body' constitutes 21.8% of total body length
1) @ zero-degree rear downslope only, Cd 0.25
2) @ 5-degree downslope only, Cd 0.233
3) @ 9-degrees downslope only, Cd 0.229
4) @ 12.5-degrees downslope only, Cd 0.23
5) @ 15-degrees downslope only, Cd 0. 236
6) @ 20-degrees downslope only, Cd 0.253
7) @ 25-degrees downslope only, Cd 0.287
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8) #7 plus a 10-degree upswept diffuser, Cd 0.262
9) #7 and #8, plus 10-degree plan-view boat-tailing. Cd 0.214
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* As the numbers reveal, the top rear slant initially lowers drag until just before the 20-degree angle, then rises above the original Cd.
* Adding the diffuser lowers drag, although it's still above the original Cd.
* It isn't until the side boat-tailing is introduced that, drag finally falls below the original Cd, for an overall 14.4% drag reduction, @ Cd 0.214.
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* This 25-degree, 10-degree, 10-degree tail relies on a series of vortices, in 3- planes, attacking one another, to burst each other.
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If it were true that the side flow on your tail WAS compromised, the above illustration might provide insight into the performance.
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I did a rough calculation using the as-shown, truncated W.A. Mair boat tail:
* Ignoring the wheels, the wake area was reduced to 15.38 square-feet ( 1.428 meters-squared. A 51% reduction.
* With rear belly pan, 'slow' diffuser, rear wheel skirts, and tail, we have a probability for a 45% drag reduction.