@Phisaac asked "What did you use for seat spacers to adjust seatback angle "
Paul, there are a couple of alternatives. One is to go with slightly longer bolts and use fender washers as spacers to get the seatback to a more vertical position. Once there, you could certainly leave them there. Make sure you use fender washers as they are wider than a "normal" washer and cover more floor area (i.e. stronger) and use them under the floor as well for a stronger resistance to any stress.
If you want even more strength, once you know the height you want from the stack of fender washers, measure that washer stack height and get a piece of 1-1/2" to 2" wide flat steel stock (Home Depot) the same thickness (stacking more than one if necessary) and long enough to fit between the seat bolts with 1-1/2" beyond each bolt. THAT will be your spacer. Don't forget the fender washers under the floor on that, too.
For even MORE strength, use the flat stock above the floor, and put a single length of that flat steel stock under the floor to sandwich the floor between them and spread any stress (from your lightning-fast acceleration or super braking power) over a wider floor area. Do that, and that seat's never going anywhere for not much bucks.