What kind of mileage are you getting with your conversions.

[RedHeartbeat]

Fbody headers are supposed to fit pretty good.

[lilred04]

i get bout 20 with my 4.8 if i stay out of it

[88RCLB]

I wouldn't advise doing the swap for mileage.

However, if you want to talk your wife into the swap the 1-2mpg increase can't hurt your argument. All the little things turn into quite a bit more $ than rebuilding your stock motor with a decent cam, so it's really not worth it to gain anything.
But, if you drive it normally I doubt you could get it to do worse than a worn-out motor that you currently have. (assuming you keep the fuel injection and don't get to crazy with your cam/stall choice).