Looking for some advice on jet sizing- Thermoquad 6139

Use TQ as is, should work perfectly. I used that carb all the time with 273's. You may have to adjust the secondary air valve so it won't bog. I'd run hydraulic rocker system simple, no adjustment, bullet proof. 340 valve springs or equivalent, you will also need kickdown linkage or cable if Auto trans. Sell me your heads and adjustable rocker gear ;-)