how many gallons is that??
the blazer tank is 21 gallons and the kit requires you to notch one bed cross support, and hangs down about even with the rear end with 2.5" lowering blocks..
this link shows how to add a intank efi pump factory set up from a newer g.m. truck/full size suv.. in the 67-72 blazer tank
1971 C-10 LS Engine Swap! | Trucks! | PowerBlockTV.com - Video
the seat thing and being safe.. that great and all.. but for the inseat belts to work, you need to run heavy floor braces to hold the seat and those in it. if you ever get hit.. the old seat mounts and or new holes in the floor won't cut it.. you'll just rip the seat out of the floor..
you can add another set of belts from a full sized cars rear seat that bolt to the floor and you'd add a 4"x4" plate to strengthen the mounting points.. and it only need to support a child/child&car seat if in a wreck,
asking the 60's cab floor to hold back up to 500lb in a wreck is asking for trouble