That's a pretty wild cam (272/284 advertised duration) to use on a plain vanilla 4.0. I hope you're planning to build it for high revs. The stock 8.8:1 CR is low for this cam and you'll find that low rev response will be sluggish. The vacuum at idle and low rpm might just be low enough to cause the MAP sensor to read high and tell the PCM that the engine needs more fuel, thereby making it run rich.
If you're going for a high rev engine (6000+rpm) I suggest that you balance the reciprocating assembly, use a set of ARP rod bolts, upgrade the timing gear to a Cloyes dual system, and upgrade the oil pump.
There is a real possibility of low vacuum/rich running problems at idle and low rpm and if this is the case, the only proper solution is a custom PROM chip with a remapped fuel curve. At the moment, only ASE Performance can offer such a chip. E-mail
[email protected] for more info.