This proposed modification of the Saturn V would have clustered seven 400klbf thrust toroidal engines in its second stage and one more in the third stage. These engines would all operate in low-pressure or vacuum conditions, so there would certainly be plume interference among the engines. The benefit of the aerospike over the bell engine in this case was actually the drastically shorter nozzle length, which would have allowed for a 1.45m tank stretch and over 40 tonnes more propellant in the second stage. The extra propellant would allow an extra 7 tonnes to TLI compared to a similarly-modified variant, the MLV-V-3, even though this vehicle used a bell nozzle engine with higher Isp.
The potential 2.8 million pounds of thrust from the engine cluster meant that a two-stage variant (Saturn INT-17) without the MS-IC was studied - and in that case the seven toroidal engines would need to operate both at low and high altitudes. While it was never built, the design indicates that such an arrangement was possible. However, with such close packing of the engines - 2.8m gimbal separation - it is likely that the low-level altitude compensation effect would be reduced.
Remember that part of the effect on truncated toroidal nozzles (ie. the nozzle does not taper to a single point) comes from a recirculation zone under the engine. The pressure within this region remains higher than the ambient pressure and essentially extends the nozzle, hence "aero-spike." As such, aerospike engines can make small efficiency gains from this trapped pressure region when in a vacuum. Presumably such an effect would not be interrupted even by close-mounted engine clusters.