Alt replacement

I agree, get a squareback since easier. For now, just short one field terminal to ground to use with your existing Vreg.

When you go HEI, you will want an electronic Vreg. You can get those for the old "high-side switching" method that are a direct replacement ($11 rockauto). You might also change to the later "low-side switching" Vreg. Rockauto sells the triangle connector if you don't do junkyards. I did that and mounted the Vreg on the inner fender next to the alternator. I was did custom wiring so had 12 V "IGN" power available nearby. You too will need IGN for your HEI module, so perhaps mount all on the passenger inner fender. BTW, get the 8-pin HEI module w/ GM coil and factory cable that connects them: 85-95 GM V-8 trucks (cars to 93). If really clever, steal the GM knock sensor and module too, like I did. Mount it in your block drain w/ a 1/4 to 3/8" NPT bushing.