Will a 1993 S-10 extended cab chassie adapt to fit under a 1950's model GMC pickup with minor modifications?

Anything will fit under anything if you try hard enough. I assume you have both vehicles and that's why you have this combination. Center wheel to center wheel on both would be my only real issue. You can alway lenghen or shorten the S 10 frame but that's more hassle. If that measurement is acceptable then set the body over the frame and weld cab mounts as needed. Cutting tourch and welder and you can do just about anything your imagination can come up with. Minor modifications depends or your level of experience and abillity.