I just ran a google image search for "schwinn frontier" to get an idea of the geometry of the bike you want to turn into an e-bike. You don't have a whole lot of room to play with on that bike. Something with a little longer wheel base would be better especially for battery mounting. Its going to be hard to keep the battery weight down low with that bike. Your options for battery mounting pretty much look like an over the wheel rear rack. Trying to mount them lower on the rear isn't going to happen without running into heal strike issues with any kind of rear pannier bags or baskets to get the weight down close to the axle height on the rear. If you use RC hobby Li-Poly battery packs you would probably be able to mount the batteries in the center frame triangle with thin sheathing on each side leaving the narrow slot in-between formed by the thickness of the frame tubes as a battery storage compartment but that will only work with the smaller thinner RC hobby Li-Poly packs it won't work with the prismatic LiFePO4 batteries which although only slightly heavier are larger physical size and bulkier and won't fit in a thin frame triangle pocket formed by just sheathing both sides of a regular bikes center frame triangle.

Your only options for mounting of a mid-drive motor are forward of the BB/Crank attached to the bottom of the lower frame tube, forward and above the BB/Crank attached to the top of the lower frame tube, or directly above the BB/Crank tube mounted to the seat down tube (not a recommended mounting position unless you have a very narrow motor drive system). Due to how tightly the rear wheel is tucked up against the BB/Crank the options of mounting a mid-drive motor behind the BB/Crank either in the lower or upper position above or below the chain-stays is not possible on that bike in addition a high rear mount behind the seat-tube is also out as well. So your mounting options for a mid-drive are more limited with that bike then some other bikes but not too badly. Obviously no mounting issues with hub-motors (there rarely is on any bike).

My personal favorite mounting positions for battery and motor are to put the weight of the batteries balanced on both sides of the rear wheel as low as possible down near the height of the rear axle and mount a mid-drive motor behind the bottom bracket either above or below the chain-stays. That is possible with bikes with a longer wheel base with the rear wheel not tucked in as tightly behind the BB/Crank. Obviously long-tail cargo bikes fit that description but also a lot of road touring bikes designed to handle rear pannier bags without heal strike issues and mountain bikes with a rear suspension system that necessitates a few extra inches of room behind the BB/Crank for the suspension linkage can often be made to work with that kind of mounting arrangement as well.