First up, welcome to the site. I’m also a fan of inline sixes, although the last time I owned one myself was back in the 70’s. I’d suggest the decision should consider how you want to use the truck. If you want to pull stumps and haul logs, or take heavy loads to the dumps the combination you already have will get the job done very efficiently. If the finished restoration is going to serve parade duty, the sounds of the manual transmission behind the six are very distinctive and rarely heard today.
For a daily driver however an automatic will be a lot more user friendly, whether behind a V8 or a 6. I like the economy and novelty of the 6 because so few people keep them anymore. A nice warmed over and dressed up 6 will always draw a crowd at a rod run.
On the other hand the reason the SBC is so popular is because it’s just so easy to make cheap reliable power. Everything costs less because there are so many around, and most aftermarket accessory and mounting kits favor the small V8. Good luck.
