Two car seats in back of mine, both forward facing. I use new lap belts brought the seats and then I have a top tether to the bolt on the back of the seat (though not sure the top tether does anything but does give one extra point.... could run that to corner bolt of rear roll bar if you have one). With your age you would just reverse the seat so it is rear facing.
I do not see any reason why you could not replace the seat belt anchor bolts with an attachment that works with Latch type anchors. I have not found any bolt anchors that work so do let me know if you find one. But as others have said...the car seats are made to work with standard lap belts.