Ummm...yeah...like that's all that was freaking around in the 70's! And, early 80's, for that matter....
Wait until that x-frame gets snagged on a downhill and flips your around 90 degrees...
X-frames don't really pivot with you, they just kind of go their own way. Which may or may not be the way you are wanting to go...
Yeah, hard to say how many miles in the 70's...days are a better count...do I get to include Philmont? In 1975 I did a 32 nighter out west...1976...one 21 nighter and one 26 nighter...1978 a three week climbing expedition...those are only the big ones...keeping track of the 5-7 nighters during that decade is impossible for me...
I had 4 or 5 big x-frame packs from 1972 to 1982. One I converted to a lashed cargo hauler for base-camp expeditions...
None I would wear today for anything other than a costume ball....or maybe to pack out some Elk pieces...YMMV...
REI is going to be very expensive for you, unless you can find something at a closeout price. However, if you have an REI near you, at least you can try on some Osprey packs. REI Flash packs are pretty good and are LW, not ULW. If you have a local outdoor shop(locally owned) for climbers or back country skiers, you can often find lightweight packs on closeout, although not ULW, because both climbers and BC skiers need bullet-proof dependability and winter carry. Measure your torso length to be sure you fit a standard pack... https://www.rei.com/learn/expert-adv...sting-fit.html
IIWY, I'd be looking to buy used or closeout. Since your x-frame is small, you may be GTG with a 50L pack. That SMD for a Benjamin is a steal. You might find a Klymit Motion 60 used for a Benjamin, new for 165. You don't need the lightest as much as you need lighter and a different suspension system. I'd say you can accomplish both for a hundred bucks on the pack. With that, you are still under $400 for replacing 2 of the big 3(pack, bag) for a savings of at least 4 and more likely 5 pounds. Plus, you get more comfort for your sleep and less weight to carry overall. I didn't give you any bag links, but here are two 20 degree bags that will work well at a decent price.
http://www.sierratradingpost.com/mar...-bags~d~208%2F Look for a 25 percent off coupon...
http://www.campsaver.com/ksb-20-down-sleeping-bag
Two things I personally never buy used...shoes and sleeping bags. Well, underwear too. But that goes without saying, right?