need help deciding!?

[Rick 91GT]

A 331 will last longer, it has better rod to stroke ratio and even though some 347 kits have a re-located oil ring they still have issues. Yes no replacement for displacement but you need to look at the nitty gritty details..rod to stroke is one of them. A 331ci motor will last longer in a daily driver like your is.

I was going to build a 418ci but the rod to stroke ratio isn't that great so I may go 408ci

Yes you could just swap uppers since they all use the same lower I believe.

The X-cam is decent, not to sure if it will pass emissions. A larger cam isn't always better, what runs good in a buddies car may run like **** in yours. What heads will be on this motor, inj, tb, MAF. You want a cam that will make the peak power where you need it, about 300 under your shift point. A custom cam ground for your motor will not only make the most power but it could also pass emissions, and have nice driveabilty down low.

My AFM cam is fairly large, but it drives and acts so nice on the street you would never know..some off the shelf cams can not do that.