1967 289

i need help ,i rebuilt a 289 for my wifes mustang ,289 bored 30 over ,had all machne work done crank heads etc installed a comp cam  had the taper checked and the old 30 over pistons .all ok built the engine  installed in the car ,broke the comp cam in ,car runs great ,heres the catch,we live in east tn and when shes going down the mnt,and lets off the gas the right side smokes bad ,as soon as she touches the gas it qiits smoking until she lets off the gas again ,i am stumped ,i am 64 yrs old and have been a diesel   mech all my life.never seen this before .anyone have any ideas thanks  

It almost sounds like there is either a bad vaulve or seat on the right bank or it could be the carb is not putting the same ammount of fuel into the engin. If the carb needle is leaned on one side and rich on the other it could be pulling more fuel in on one bank than the other during deceleration... either way its not suppose to do that so I'd take it back to the shop you had it done at.  

What you descibe typically points to the oil from the valve guides sucking oil down the valve shaft(highest engine vacumn is during decel). So what you need to know is:
Is one or more of the guides oversized or is it just a valve seal problem? I'd be more inclined to think valves over fuel because the IM distributes Air/fuel mixture to all of the cyl's so both exhausts should smoke if it was carb related. jlg  

Did you add any custom valve covers, ive seen with some chrome covers there is no oil shield at pcv and it will suck oil off of rocker arms badly?  

Umbrella style valve seals sometimes don't work so great. I swithced over to circular seals when the heads were rebuilt and eliminated oil loss down the valve guides getting sucked into the combuation chamber(s).
You might consider checking your seals out.