This should have already been asked, what do you plan to do with the car and what is your budget?
The LT1 is a nice choice because is it a bolt-in swap, has all the advantages of EFI, is a relaxed street motor for daily driving, and is a strong motor right out of the box but with the potential for gobs more power.
A carb motor will be cheaper and easier. It can be built for big hp numbers but it wont have the same level of refineness at those numbers as an EFI motor.
The LS1 is a step above. Lighter, stronger, silky smooth with even more potential. But this one will be the biggest challenge and cost.
As far as what displacement that's simple. Cubes = power. The whole argument that a smaller displacement motor will rev up faster has had a lot of holes shot through it. Any motor can be built to rev if you throw enough money at it.
Between the LT1 and the LS1 the Ford swap has lost a lot of it's allure. While it's true that the Ford is lighter, a complete f-body (aluminum heads) LT1 is not that much heavier and has the distributor in the front out of the way of the hood latch, like the sbf. The Ford has fitment challenges as well, namely the pan that has been mentioned, and the drivers side header to steering shaft clearance.
About weight gain. Even worst case senario we are not talking about making a sports car into a truck. Because, in the jtr position, the motor sits lower and farther back in the car. The difference adds up to having an average size person sitting in the passinger seat.
Whata ya wanna do? Whata ya wanna spend?