@NookShneer is right about bolting in the back HJ61 seats. I bought our set in early 1991 from SOR when wifey became pregnant with first kiddo. The easiest solution was to let the Toyota dealership body shop install the rears. Quick, easy, affordable.
That's not the only feature that changes. There are plenty that many of us regard as pluses to a split back. The flexibility to have only one deployed is likely the biggest plus. Besides, your munchkins better be pretty small to fit three across comfortably in the OEM bench. If you have two child seats at any time, maybe a dog will fit in the middle.
Although I sold my HJ61 front seats to
@CenTex62 and have two BMW seats standing by for install when my 62 returns from paint this weekend, I will keep the split rear seats and have them re-covered to match the BMWs. I do have to work out a solution to secure the back seats to the front when they are tumbled forward. Each HJ61 front seat has an eye in the floor bracket in which a male hook with retention strap secures each tumbled forward split seat. Easy mod.