Hmm. Those would have to be from a fairly new Mustang, as the older ones have always bolted in/on. look at the trim plates on he side of the seats. take one off and look for a part number. THAT will tell you what year those seats started being used. E=8, F=9, G=00
I looked around at different year seats on the web... I think they came out of a 97+ v6 Stang or something similar (DEFINITELY MUSTANG THOUGH and also explains why the other guy didn't have the balls to do the swap and sold em for $100!!! Lol)
As was said, MOST of the seats are a direct fit on the tracks...but sometimes you need to do slight modifications...and, as you've found out, those front bolts don't directly line up...
But, you can always just drill new front holes (if the back ones line up) and brace them with washers or other metal (usually under the floorboards needs it more than above). Main thing is to have enough "extra" metal on both sides to prevent them from pulling out in the event of an accident.
MOT in Canada requires grade 8 bolts to secure seats to the floorboards...for a good reason. Make sure that whatever hardware you use (I know, it's a race truck) is appropriate for local laws if you do plan to have it roadworthy....
All I did was drill/tap/helicoil the front ones... They locked down DUMB tight!
Then, for the back (which were also wide!) I drilled a hole the same size as stock, ran up to the local hardware store and got me some long enough bolts to go through the floorboards and some BIG A$$ washers! I simply just slipped thru the hole I drilled and CRANKED down (with TWO breaker bars as hard as I could.... Enough to FLATTEN both lock washers BEHIND the BIG, THICK washers!
They're going NO WHERE, and it's ALL Grade 8 bolts/nuts/ext....
My passenger seat is GLORIOUSLY amazing to sit in, compared to the STOCK seats... This needed a little bit of modification but not much by any means. Anyone could do it! The payoff is awesome too. It survived a wreck last night (yes I wrecked ny truck, but it doesn't seem to be bad AT ALL.... God Willing!
That said, the way I did those seats... You couldn't EVER rip them out by hand, or by force. Period. They're legit, and I didnt even do much! Just drilled some holes, used two helicoils and two long @$$ bolts for the rear... As I previously stated.
You just gotta GET UP AND DO IIIIITTTTT!!!!