I'd also be careful of the rubber unions. When the ground starts to settle they could be pulled apart by tension induced in the pipe. I've made 'custom' pvc adapters out of pipe or stock couplings by heating the pvc and enlarging it to the size you need by working the hot pvc over the size pipe you are adapting to. You could probably find a bell reducer that would adapt to the wye and you could form to the diameter of the SDR 35. Then you could glue it all up.
