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if its factory engine then ya 7.3l. ive gotten a few of these trucks that the engines have been replaced. easiest way to tell is to look at the oil filler. if its molded into the ip gear cover then its 7.3, if its bolted to the front center of it then its 6.9
If that hadn't been swapped either... At this moment, there is a 7.3 timing cover on my 86 6.9... There is no surefire outside appearance to guarantee either.
It doesn't matter on the motor, more the transmission. If 5 speed ZF, you will have a dual mass factory, so you then have to decide if you want to stay with the dual mass, or go to a solid conversion.
And yes, as of 88, all factory installed diesels were 7.3 IDI.