Hey Homebru,
I thought more experienced bowmakers than I, would have jumped in by now, but since they haven't......... Picture shooting the bow. The side of the bow that faces you (the closest side) is the belly. The side farthest away from you (the side the target's on) is the back. To reduce weight, and the way you tiller a bow, is to remove wood from the belly.
A WARNING! Go
very slowly removing wood from the belly. Use a scraper, not a rasp nor a blade - or you will remove more wood than you need. The bow pulls 70# now. You don't have to remove a lot of wood to drop the weight, in fact, you could do it with sandpaper, it would just take you awhile.
Remove a
very small amount and check to see how the limbs bend. Repeat until you get to the poundage you want. You don't want to create 'hinges' (weak spots where the limb bends more than other places). This is where the bow can break.
I can't emphasize too strongly about going slow. 'First time bowyer's mistake' is making a bow with too low a draw weight. The first bow I made went from a 200# draw to 32# by breathing on it!!! OK, I'm exagerrating a bit, but you get the idea.
Actually, I just thought of an idea- check
this out. Scroll down the page to Archery Archives - you will find more information than you could possibly use.
Alternatively, spend some money and buy
Traditional Bowyer's Bible, Volume 1. This will tell you all you need to know and a lot more. The 3 volumes of Traditional Bowyer's Bible are unparalleled in bow making instruction (from a book). They also have sections on glue making (primitive), flintknapping, arrowmaking, and an excellent section on cordage making.
Tillering is the absolute most difficult part in bowmaking, so be very careful. The fact your father made it for you means it has tons of sentimental value. You wouldn't want to blow it up.

In fact, in view of this I would suggest you make a bow, or maybe bows to learn the technique and then you could more confidently retiller your dad's bow.
My 2 cents - don't change anything. Keep it just the way he made it. I bet down the road you would be glad you did.
Doc