MGZT260 said:
If it's power you are looking for - beyond what a back to factory spec B engine will do then some thoughts:
*cross-flow head with appropriate weber carb
*super charger
*"C" six-cylinder engine - you know that will fit will minimal parts which are probably all available form Moss or VB, coming up wit hthe engine is trickier
"You know that will fit" ???? Are you serious? The factory couldn't put the C series engine in there without changing evey internal body panel forward of the firewall, AND changing the whole front suspension to a completely new torsion bar set up. I think the old saw "Don't try this at home" surely applies here.
Couple of other things. If you fit a crossflow head with 4 intake ports no one but an idiot (they are out there) would fit 'a 'Weber carb', which indicates a single downdraft, although many have fitted two sidedraft Webers (even though a pair of SUs is the better choice for street with that head).
The supercharger, however, is an excellent bolt on performance improver!
The Miata engines are an excellent option, but with a lot of do it yourself skill and time. A friend has done one MGC and at least a couple of MGBs - here's a pic of his MGA:
The OP shouldn't be put off the V6 route if that's what he wants to do. The kits are available for reasonable money to fit one with T5 trans to an MGB, and there isn't that much extra that needs to be done to relieve a couple of spots in the tighter tunnel on the early cars. Get in touch with one of the vendors to get details.
The minimal mods needed to put the V6 in an early MGB pale in comparison to the major work needed with the Miata transplant.