['68OrangeSunshine]

The 307 is an orphan. It gets very little love or respect from the engine-building ''Hot Rod'' community. The 283s were revered for their spunk and surprizing torque, the 327s were legendary for being able to get turned into race machines. Either of those would get enthusistic support for a rebuild project. I did once talk to a wizened old mechanic in a long-gone junkyard in South Tucson. [C. 1994, I was buying a GM HEI for my '67 K/10 Suburban]. He had a 307 in his Camaro, that he'd rebuilt with some snakey tricks. It was so hot, he wouldn't let his goofy teenage nephew drive it... Some weird trick with bevelling an angle on the heads. IDK the theory there. Long Gone Technology now.
Anyway, unless you're going for Concours d'Elegance Originality points for having the Original Engine [''Numbers Matching'' does not exist for trucks. They were expected to use multiple engines in their service career] -- best advice is to get a Chevy 350 Mr Goodwrench Crate Motor. It will have a 3 year, xx,000 mile warranty. Fit perfectly, and give you 43 more cubic inches of power. Also economically priced. And probably less than a thorough rebuild of the 307.

Shop around. I got $100 off the quoted retail from Matthews Chevy at O'Reilley Chevy * for having a ''good Irish name'' -- Even though they were 10 miles out of the way and crosstown. [* No relation to the auto parts chain.] I have a crate 350 in my '71 GMC Jimmy. 21 years old and still running strong.
Jegs and Summit are good prospects.