You can take the motor out of the 1990 truck and put it in your 85. Just take the intake manifold off your truck and put it on the new motor. I took a 1995 350 and put it in a 84 suburban the same way.
As Farm boy pointed out, your manifold off the 1985 motor will not just bolt onto the 1990 block. You will have to modify the 4 center bolts (2 on each side of the carb) to bolt up to the 1990 block. You can buy an aluminum manifold that will bolt right on to the 1987+ blocks (center bolt valve covers)for about $200 ( just a guess). Other than the fuel pump I mentioned earlier, most parts will swap over. The 1990 motor is a 1 piece rear main block, so use the oil pan & flex plate off that motor.
There is nothing wrong with keeping the TBI, just more work, and probably allot more $$.