Mark,
The IR's from Roush will be fickle time to time. I have done several engines both FE's and small blocks. The ones I have completed need to be tweak at or around 1k miles. Is it worth the trouble? You will have to ask yourself that question. All my customers love this setup. They are beautiful and perform very well.
With your quesitons on the 427's or 402's. Here again you have to ask yourself how are you going to drive this car. If you are going to lug around at low RPM in town, you may want to consider the SR/TW version of either of 427 or 402. If you drive more spirited at higher RPM you may want to stay with the "R" version. I perfer the R version because I like to drive a little more aggressive around town or on the highway.
These are my thoughts on this. It may not be for everyone else.
Bill