Welcome! Glenn has given you great advice and some options, and while you may think his costs are way out of line he's been doing this stuff for a long time, has the tools & experience, and tells it true.

At 21 I expect that your assortment of tools is pretty basic, so rebuilding your engine yourself is going to present some challenges that you need to consider. Engine hoist/Cherry picker - you can buy one at Harbor Freight (HF) but then you've got to store it so borrowing one or renting one makes sense. Once your engine is pulled you have no truck, so you need a way to get it back - gotta have a plan. Engine stand? HF again, or maybe a buddy has one you can borrow for a month or two? A roller dolly is nice for the tranny - cheap at HF but lots of cheap $$ adds up fast. Do you have a safe & secure place to work where you can park your truck and have your disassembled engine for a couple of months without anyone complaining? Even if you live with Mom & Dad their patience may wear thin after a week or two, especially if the neighbors, or worse the HOA are calling wondering why the truck hasn't moved in so long.... Live in an apartment? Even worse! Tools, let's cut to the chase and say you probably need to add $500 or (likely) more to your budget for things you don't have but will need for a DIY rebuild.

I believe that you'll be way, way ahead of the game if you go the crate engine route, or perhaps a salvage takeout package from a later model that gives you engine, tranny, ECU and harness from a reputable supplier. Most important is to take time to really think through everything, decide on your path, then plan, plan, plan. Once you have your plan in place commit that you're not going to deviate when your buddy says, "Hey, how about we...." because changes are budget busters!

It's great that you're wanting to freshen up the truck, especially since it's been yours since 16! Take a look at this thread that Glenn updated not long ago - 1971 c-10 How about a picture or two of the truck as it is now?

Glad you're here, looking forward to following your project no matter which way you go!