JMG,
#1 can you do labor or do you need a mechanic?

#2 what makes you think you need a cam?

#3 you can get a edelbrock 600 for around $300
and a used LD340 or 318/360 performer for another $100 used
plus another $20 for gaskets. so you are looking at $400+ just for the parts
to do a 4 barrell.

#4 windshield where is it leaking? often when they replace the seal around the
windshield it can break but make sure it is the seal that is leaking.
if it is then you probably have rust in the frame that will need attention as the odds are
its not just the seal.
if its coming from under the dash then i would lean more towards wiper pivots/seals
bottom of this page
WINDSHIELD WIPER
that are leaking. it is a common problem on old mopars.