First question, MGB engines are not that hard to get and if their in average repairable condition, I wouldn't pay over $200 to $300 for one depending what accessories are on it.
I forgot the exact year the change was made, but around 1975, the transmission tunnel was widened on the MGB and remained that way up to and including 1980. This allows more room for a V6 or V8 conversion. Small block Fords, Chevy's, Buick's and even other foreign makes have been installed with success. Even though their not cheap, the British Rover V8 is a popular conversion and for some, keeps it British. 5 speed transmission conversions are also available, but expensive. Don't worry about asking questions, that's what this forum is for. We all ask questions at times, as there is always a question that needs answering and the amount of knowledge you'll find on these forums is mind boggling. Hope this helps. PJ