Sorry, I realize now I was thinking you were puttin the 88 in the 89, but it's the other way around. So it would be slightly easier since you could keep the '89 bell housing (I think) and just plug the hole it has for the CPS. I may have made it more complicated than it needed to be anyway. If you could bring over just 88 engine from the 88 and use your '89's injectors, distributor, IAC, sensors etc, then I don't think you would need to change the wiring harness and computer either. The blocks, heads and intakes are the same. I think the exhaust manifolds are different if you were thinking of bringing that over too.

'89 740 n/a cars also had LH 2.4 and the 240's kept it until 1992 I think. The AMM with part number ending in 16 is the quick way to tell if you have the whole car to look at.
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Andy in St. Paul - '91 745 227K mi, '91 745 210K, '90 744 194K, all Rex-Regina; '86 745 GLE M46 230K; past 240s