Bonjour!
I understand your problem, gas is very expensive over there in Europe. The Thunderbird was made in a time when gas was cheep so big motors were best. Not so now.
Smaller engines of today have the Horse power (PS) but not the Torque that you will need to get that big car moving. I think you will be very unhappy with your results after repowering the T-bird. Also, if you replace the engine with an engine that was not used in that type of car, the value of the car drops quickly. Others will see it as "not original" and be unwilling to pay the best price should you want to sell it. The engine compartment is big enough to put just about anything in there so your choice of engine is great, making it all work together is going to be the hard part(hooking up the transmission, cooling lines, etc.). I have no information for you on re-powering a T-bird down to a smaller motor because...will, it just isn't done.
It's kind of sacreligious here in America to do that. If you want to know how to get 450 hp (ps) out of that Glamour-gal, I can help.
A suggestion. You may want to spend the money on another car to drive everyday that is cheep to gas up and save the T-bird for Sundays and special occasions, that is what I do. I don't like to put meaningless miles on my Glamour-gal.
If you want to replace the engine in your Glamour-bird with a small block of some type you may not like you bird any more.
Let us know what you decide to do and if we can help any further. Take care from America!
Good Luck (Bon chance)!
Lad