John, several people have adapted GM 3 speed blowers to the MGB and Midget heater box with good results. Here's a link to how one person did the job. http://www.mgexperience.net/phorum/read.php?1,519704
With the addition of additional electrical loads such as a more powerful blower or electric engine cooling fan you may want to consider upgrading to an alternator at some time as well, but the generator is fine as long as you don't do a majority of slow speed driving where it wouldn't charge as well.
'73 Midget (V6)
'59 MGA (I6) under construction
"There is a fine line between a hobby and mental illness"