Why dont you just build your 4.0. Do a stroker of somthing. I am going the v8 route with my truck and it is being a pain in the butt. You probley have more room then i do though cuz the I6 is longer then my 4cyl. I went the 350 route just cuz you can build as much power as you want just depends how deep your pockets are.
If i were you id look into building your 4.0. Maybe do the 4.7 stroker and all the other things before you do a v8. Little easier then a v8 swap.