Randy,
Addressing your concerns about a RB-49 with six J-35 engines atop the rear of the wing. The R-4360 engines, their gearboxes , turbos, extension shafts and contra props were massive,vibrating, torquing monsters. The four in the XB-35, from which the XB-49 and the YRB-49 were modified on the factory floor, weighed, installed, 4080 pounds each. A total of 16,320 pounds. The proposed six J-35 engines weigh 2400 pounds each; six totalling 14,400 pounds. Add nacelles and the weight (and moments) is about the same as the recips. Engine mounts for the smooth running jets-not complex chrome-moly space frames, but largely aluminum sheet doublers riveted to existing ribs and spars, weighing less than the extensive internal wing ducts required for the in-the-wing jets. Compared with the engineering effort required to go from the XB-35 to the XB-49, the above the wing engine modification is- to use a word I've seldom used- trivial.
To summarise, for no net increase in empty weight, we have a more powerful aircraft with greatly enlarged internal fuel tankage, incorporating proven engines, mounted on fins which would improve lateral stability and compared with propellers and underwing jet nacelles, or potentially resonant leading edge duct openings, would further reduce RADAR reflectivity. Also performance enhancing jet energy input to circulation over the wing trailing edge ( boosting the Lambda line integral term which stands for circulation strength in the Kutta-Joukowsky equation: Lift= density (velocity) Lambda) to reduce wing angle of attack and consequently drag at any comparable set of flight conditions.
I can't put a number on the improvement without a lot of non-existent data for exhaust viscous mixing with ambient air etc. To summarize, the proposed mod would be far simpler than the actual factory floor modifications accomplished in the B-35/49 series IOTL and produce an aircraft ideally suited for long range high altitude photo-recon.
Dynasoar