Rebuild

To answer a coupla questions: Boring won't add a lot to the performance; so it should be done just to striaghten up the cylinders that may be worn beyond usable limits. Usable limits means that the psitosn are too loose and they slap around and the rigns flutter going up and down rather than staying flat as they should. You only know this by:
- a poor compression test result that improves when you add oil into the cylinder and retest
- engine is disassembled and the bores measured and a judgement made

As for magnafluxing....you do it for use if you rare realy going to rev the engine a lot. If not, then it is less necessary. It is done just to detect small cracks in the surface that could become big cracks when revvd high. So, if you are keeping the revs down and not racing this engine all the time, then you get away with not doing it. I have done a lot of high revving race einges and never had things magnafluxed and got a way with it; that was on engines that had forged cranks as stock, and the /6 does have a forged crank prior to '77. (And maganfluxing does not guarantee against failed engine parts!)

Some basic balancing of the piston/rod assemblies should be done if you change pistons as part of a re-bore, IMO> But again, if this is not a high rev engine, then it may not be as needed. I personally would spend $$ on this before magnafluxing.

Speaking of revs.....I have not modifed or raced the /6 but all indications are that it si not a godd breathing engine. But, if you focus on mods that keep the engine revs low for the desired pieces, then any serious porting (expensive) can be dispnsed with and just some simpel port mathcing and port clean up done. (But you will probalby be OK even without that low level of work). This would point me in the direction of focusing on a low RPM, torque cam selection. If you are after just some fun on the street, then that probably works. And you get better gas mileage too.

Be aware of the fact that early /6 engine were solid lifter and later were hydraulic lifter, and that changing involves changing the rocker shaft.

For what you seem to be saying you want, then a simple cam, carb and exhaust upgrade will get the smiles that you are seeking. Keep the revs low and the torque high, and forget about the ultimate high end HP numbers.

I really would follow the recommendations of looking at slantsix.org ....

As for cost and who to use, it can be all over the map. Just be a wise shopper as for anything and go slow since you are new to this, and get a reasonable level of details on what is to be done and the approx cost for each item. I would stay local, at a place that you can easily drive to. Visits in-person rather than over the phone are invaluable.