The stock 300 pistons are not up for more than 6 psi, and high boost will kill it dead. If your engine is in good stock conditon, but rebuilt with just good rod bolts, bearings, rings, crank, valves, better than stock cam and good quality bores, you can then add a good set of 390 pistons or very good aftermarket pistons. An EFI 4.9 engine will have less bore wear than a 300 carb engine, but any babied low mile 300 or 4.9 should be able to rebuilt without having to rebore the cylinders, and line bore the crank tunnel and refit the cam bearings. Just a hone out, and check for out of round and wear, you can do a rebuild fine.
The Aussies run low 11's on 4.0 OHC and 250 x-flow engines that are just stock engines from smashed taxis or old company cars. They rip them appart, add good replacement blister pack bearings, gaskets and good pistons and rod bolts and valves, then boost the living daylights out them!!!.
They are in cars in the 3500 to 3800 pound level, with autos and all power options. Some still run gearing in the 3:1 or even 2.77:1 range. It's a delight to watch these old 1980's dinosaurs kick ricers butts.
With your 300's, you've got another 20% more capacity, a better engine block, crank, rods and bore to stroke ratio. A 4200 to 4600 pound truck could do low 11's with very close to stock gear. Heck, a rebuilt AOD will see 850 hp or a C6 will see 600 hp without any major issues. You've got 9, 9.75 and 8.8 inch diffs the envey of the western world, and any ratio you want, you can get. We don't have that luxuary in NZ or Australia. Nor do we have Offy intakes, dual out cast iron headers, nor the space around the engine to run all the turbo plumbing like you do. We have to fight the confines of spring towered unibody frames, and make up exhasts out of boiler tube and find old C4 gearboxes with rare six cylinder bell patterns. You got it made. I hate ya!
Any engine costs. The trick with a turbo 300 is that its no cheeper than V8's but its like manic street prechers. They are no fun unless they are on fire and being moved by some irrestistable force. Anyone can buy and abuse a V8, but a rebuild six with a hair dryer is burning with fire and brimsotne, not pooring out the same slurry you get from everyone else is doing.
And turbo charging is a American as Covair Spyders and Olds Jet fires, and these days, there is more expertise around. Copy Does 10'S, and keep moderate restraint on the bills. Spend on rebuilding things yourself, and gain a skill.
One other thing. A 351, 400 or 460 will always use 351, 400 or 460 cubes of fuel. A 12 pound boost turbo 300 will cruise like a 300, then behave like a 580 cube engine when stired.