Congratulation s on getting it running. Just in case you did not notice, that spring does not hold the belt tension. The spring is only used in setting the initial belt tensioning. After the two tensioner plate bolts are torqued, the tensioner spring does nothing (except maybe as a backup if something loosens). If it really is too loose, I suspect the initial tensioning procedure could be improved.
I have those parts used from 71-72 engines, should be the same as on 73 engine. If wanted, just PM me your mailing address and name for use on one of those small USPS fix rate boxes, cost too small to hassle with (no charge).