First off I dont think you'd be able to squeeze those 60" leafs in and if you did it would make a horrible ride.
The rule with leafs is you want the perch's to be the same length apart as the spring eyes, the spring bow will give any lift.
Springs should be measured eye to eye straight across, not eye to eye across the spring.
Very few springs are flat or reverse arched, even lowered trucks try to stay away from flat and reverse arched springs because the ride is to stiff.
Chevy did use flat springs and springs with a slight reverse arch on the front of late 70's to mid 80's trucks and they had stiff suspensions with springs that wore out real fast.
Look at aftermarket replacement springs for those trucks, almost all of them are made with at least some arch to them, most times if you replace the front springs you end up having to replace the rears also because the new front spings will give the truck a pre runner look.
Also remember 3/4 or 1 ton springs will only change the spring rate, it wont change the fact you have a lighter duty rear axle under the back end.