NO... don't even think about it. The retainer has to be a snug fit in order to keep the transmission input shaft on the same axis as the crankshaft.
I had a 4.34 retainer and a 4.80 belhousing once. The simple fix was to have a buddy make me an aluminum adapter ring with a 4.80 OD and a slight interference fit onto the retainer OD. Then I just pressed the ring onto the retainer and bolted it on the trans. You could do something similar with the 4.80 to 5.125.
Check the runout either way, it is common to have to dial in the bellhousing alignment with offset dowels...
