Chris, I concur with Molakules comments about the 15w40 but in addition, you could even use a 20w50 as that engine's design was for straight wt oils and only 30 and 40wts were common at that time, therefore, both the 30 and 40 are thicker than a 20w50 on startup and since you are living in Dallas, I know from personal experience living in Tyler 88 miles east and Arlinton, that summers can get hot. Keep in mind that a thicker oil in an older engine will act like a cusion and not cause the wear as a thinner multigrade oil, so If you do use a multigrade, make sure there is a good barrier additive in place.
Also During a cold winter, if using a multigrade, I'd consider dropping back to 15w40.
Quite frankly, if it was my car, I would lean more to running a straight wt, but the other options are reasonable.
in my 95 ford I use at present the Schaeffers 15w40 which is the new SL for gas and CI rated for diesels. This along with the chevron delo and amsoil have higher barrier lube properties and will provide the protection with what the older cars called for, but I don't know the api's spec's for if it is SL or not. They could answer that better.