Crate 5.3 Vortecs seem to be in the $2500 range, from my imprecise Google search. A Mark's of Australia kit is in the $1300 range, and a good Toyota 5-speed is around $2500. Since you have a 74, you already have the right transfer case, and if it's in good shape, run it as is. So that adds up to $6300 in major hardware, not including shipping.
The Mark's of Australia kit is pretty complete. It does require welding on a scab plate on each inner frame rail for the front motor mounts. The rest bolts right in. No hacking of the firewall is needed. The kit does require a Chevy flywheel, pressure plate and clutch, and instructions are given (and should be heeded) for which ones to procure. The stock Toyota hydraulic clutch setup is used. If yours is in good shape, reuse it. If not, add that to the list of minor parts.
Radiator. If it is in good condition, use it. I'm running the stock radiator with my 350. I'm running the largest flex fan I can, but it's a standard V-belt setup. If you're running a serpentine setup, if I recall right it will need a reverse rotation fan, and choices are fewer for size. An alternative is an electric fan, and a popular swap is the Ford Taurus setup.
Other minor parts. Air cleaner. Throttle linkage. Maybe gauges. I'm not a fan of block hugger headers with the stock steering box. Clearance is way too close. In fact, two of the tubes have to be slightly dimpled. Rams Horns are better. Figure in the cost of an exhaust system. I'm running a single exhaust system with a large Flowmaster.
So that's, what, another grand or so in minor parts at a minimum? So that's around $7300 in hardware.
Then there's labor. I have no clue what a shop would charge for such a conversion. I have all the paperwork from mine, which the previous owner had done, but that goes back a few years, so the prices are out of date. A figure of $12K was cited above for the whole job, so that's around $4700 in labor ig you go with the $7300 in hardware estimate. Shops, though, usually charge more for parts than what you can buy them for. Some mark them up a fixed percentage, others go with MSRP. So this will tend to distort the hardware figures if the work is farmed out.
As for the $38,000 figure, that's simply ridiculous. If I was quoted that, I'd take it as simply being blown off by the shop. Maybe they get some rich clients who have more wallet then common sense.