What year is your MGB? Rubber bumpered models usually require less engine bay/transmission tunnel than the chrome bumpered models.
Buick/Olds/Pontiac/Rover (BOPR) is what your after for the factory-style block.
Here's D&D's website:
https://www.aluminumv8.com/
I used a Rover 3.9 litre and had it bored & stroked to about 5.0 litre.
And don't rule out the Ford 302. In all likelyhood, it may be less expensive oin the long run to use this motor.
I've also seen a couple of LS3 conversions.
You'll probably want a BorgWarner T-5 transmission. There are Ford and Chevy versions as well as V6 and V8 versions. Do you homework for what gear ratio will give you the performance you desire.
I used a T5 from a late 80's Camaro V8. It has a 0.63 final drive gear.
Your stock MGB 3.9 differential will most likely be strong enough for a stock BOPR or Ford 302. If you beef it up, you may want to look at a Ford or Dana differential
I used a Ford 8" from a late 70's Granada and had it narrowed. I also had an Auburn 3.27 positrack gearset installed.
You might want to peruse
https://www.britishv8.org as it has a wealth of information to be discovered.
Safety/Faster!