Your 1981 300 inline 6 uses a typical cast-iron 1-piece log type exhaust manifold and 1-piece log type intake manifold that are bolted together under the carb plenum. Check all gasket sealing surfaces including those for the riser gasket beneath the plenum. If any are rusted/pitted but not so bad that they cannot be milled a small amount to make a smooth sealing surface, take care of that before joining them together.
After you have replaced the under plenum riser gasket between them and joined your intake and exhaust manifolds together (loosely at first)...... Trial fit the manifolds
without intake and exhaust gaskets to your head. Snug up bolts (or studs) holding manifolds to head evenly to 10 ft./lb.
Then, tighten the bolts (or studs) holding intake and exhaust manifolds together. This is necessary to align them properly with one another.
Remove manifolds as a unit and look closely at the gasket sealing surfaces of both manifolds where they bolt to cylinder head. Again...... Check both with a metal straight edge or framing square and if they are uneven, warped, rusted or slightly pitted...... Have a machine shop do a skim cut to true them. And do this with them bolted together as a unit just as you now have them.
Same thing (if needed) to corresponding gasket sealing surface on cylinder head. Check for rust, pitting and make sure no warpage. If those surfaces are straight and true and not pitted, any good aftermarket or OEM style gasket should seal them if you torque them properly and then come back the following day and retorque before starting engine.
Also recheck torque following first heating/cooling cycle after running engine and you should be good to go. Begin at center of intake/exhaust and work concentrically from center outwards in 2 or 3 incremental steps same as you would when bolting the head to the block. 20 ft./lb. should be sufficient torque for fasteners holding manifolds to head on these engines.
I have disassembled a few OEM installations (both inline and V-type engines) where no gasket was used at the factory exhaust manifold. And I have read Sanderson's installation instructions stating that a thin coat of high temp silicone RTV can be substituted for gaskets on their exhaust headers.
However, I am uncomfortable doing a gasketless reinstallation personally, so I have always used a good quality aftermarket or OEM exhaust manifold gasket going back on. And if its the multi-layer steel core laminate type exhaust manifold gasket found in most Chevy gasket sets...... Or even a high end aftermarket dead soft aluminum or copper gasket...... I coat both sides of the gasket(s) with Copper Coat when reinstalling the intake and exhaust manifold on an inline 6. Whether it uses a 1-piece gasket for both or separate intake and exhaust gaskets. Same for donut gasket where exhaust pipe connects to exhaust manifold.
Exhaust studs and nuts where exhaust pipe bolts to manifold...... Cheap insurance...... Use new if you are going to keep the truck for some years more. Me...... On a keeper, I like heat treated carbon steel studs (grade 5 minimum, grade 8 if you can find them) and thick brass nuts as used on older cars and trucks and having an additional thread or 2 for increased thread engagement. But that's just a personal preference. If you are considering stainless steel fasteners, read this beforehand with respect to galling and consider brass nuts rather than stainless nuts on stainless studs for exhaust......
https://www.boltdepot.com/fastener-info ... lling.aspx
Hope this gives you some ideas,
Harry
P.S. >>> Here are gaskets for your installation from a couple sources. You can probably find (or order) the same gaskets at your local auto parts store, as well......
If you opt for separate intake and exhaust gaskets......
1. Fel-Pro Intake Manifold Gaskets MS901571......
https://www.summitracing.com/parts/fel- ... /make/ford
2. Mahle Intake Manifold Gaskets MS16040X......
https://www.summitracing.com/parts/mah- ... /overview/
3. Fel-Pro #1447 exhaust manifold gasket set......
https://www.summitracing.com/parts/fel- ... /make/ford
4. If you opt for 1-piece intake and exhaust gasket plus separate riser gasket......
5. Remflex #3005 1-piece intake/exhaust gasket set + riser gasket (not the cheapest, but a very good gasket and worth every penny) I am using the equivalent Remflex 1-piece gasket for my 292 Chevy inline 6......
https://www.summitracing.com/parts/rfl-3005
http://catalog.remflex.com/FORD_Header_ ... p/3005.htm
6. AP Exhaust #8717 or EXD #HG017 Donut gasket for exhaust pipe......
https://www.summitracing.com/parts/aph- ... /make/ford
http://exhaustdirect.com/HG017.html
Note: Not sure if you can get them anymore, but 390 Ford engines in the early 1960s used a donut gasket that was identical dimensionally to the one used by GM for 2" exhaust except that Ford made their gasket from a soft metal (looked like aluminum) and these would last forever on mid-'50s and early '60s Chevys. If this is the type and size donut gasket your '81 300 Ford inline 6 uses, see if you can find a couple of the 390 Ford soft metal donut gaskets and hang on to the 2nd one as a spare. They really were one of Ford's 'better ideas'.