68 was the only year for the 145 B18. Contemporary road tests described it as underpowered. It's not unlikely to have been upgraded to a B20 already at some time in the past 38 years.

Many other differences from later cars:

68-70 had two-piece rear side windows with a hinged rear section. The latch is glued to the glass, just like the vent windows, with all the problems that entails.

Everything in the front end is smaller than the later cars---ball joints, bushings, tie rods, brakes etc. It's not too hard to upgrade to a 71 or later front end if you can find the parts. Four bolts hold the crossmember on.

Rubber floor mats instead of carpet, different vinyl on the seats, similar to that used in 122s.

Windshield mounted with a rubber gasket instead of glued in glass.

Dashboard more prone to cracking than later models---material changed in 70 or so, the shape stayed the same.

B18 cars originally had a generator.

Probably a lot more I can't think of at the moment. It's been about 25 years since I had a 68 (my first Volvo)