Classic/Antique Car Repair: rear end, 1965 ford falcon, trans work


Question
I have a 1965 Ford Falcon originaly 6 cil. and standard transmission.I put a 302 engine and a C4 Automatic Trans.I know that model came with 6 and 8 cil.From factory because I have another with 8 cil. an automatic trans.original.
The problem I have after I put the 302 engine instead of the 177 6 cil. When I go in third gear the engine comes forced like if needs a 4th gear.I been thinking about rear end problem because the car still have the original 6 cil. rear end.I know that model came with 2.8:1 and 3.0:1 ratio on a 8" ring gear.I have been asked to differents mechanics and some says there is a problem and some say dont`n. Do you think it is the problem.
Another question is if can exist any risk to drive that car in that condition,because I wrote this question to another expert on old cars and He say "Don`t drive that car"
What do you think??

Answer
IF" you installed a C4 transmission did you also install a kick down rod?  This trans has a manual not an automatic way of down shifting when it hits higher rpms.
If you can give me more info I could help you.
What year is the motor and trans?
Is the motor and trans from the same car?
Were the motor and trans ever worked on?
Did the trans work CORRECTLY in the last car?
At what speed and rpm does this problem happen?

When you wrote the other expert did you tell him everything you told him and told me everything ?
Until then I would take the advice of the other expert and DO NOT drive the car.
Answer as many of these questions as you can before you wtrite back to me.