questions: 3.8 to 4.2 swap

hi
i am planning an engine swap in the near future.
at first i was unsure whether to go with a 5.0 or a 4.2
i have an 89 tbird with the 3.8....tho i have removed a lot of weight
i feel like the 4.2 would be an easier swap
few parts to buy
i can pick one up for 200 bucks
can use my computer
no new k member
can my rear end handle this?
i was told before that it should stand up to 300 hp if i am not too hard on it
i saw that this motor makes upwards of 200 hp and over 250 lbs tq
and it will basically drop right in
has anyone out there done this???
and are you happy with it?
basically i am asking is it worth dropping my v8 dreams?
because being speedy and keeping my 20+ mpg sounds awesome.
and spending less than 500 bucks for the whole deal
i know some of you will be like always go v8
no replacement for displacement
but theres no replacing gas money either.
so..... im just opinion seeking
give me your thoughts

thanks
justin