to do the gasket yourself
you will need (based on my own slightly different motor, with similar fixings in different positions and a lot less access space)
the 1/2 inch sizes fit my car its australian and the motor is a hemi6 not a slant 6
your car may have 7/16 sized nuts
1/2 or 7/16 inch open ended and ring spanner double ended long-ish 11/12/13mm would do at a push....
1/2 inch or 7/16 box spanner like plumbers use. buy a set you can usually stack them up to make one long long box spanner and use a socket on the fat end to adapt it to your normal drive..these cost little because they are usually cheap rubbish, you might need it for the underneath position between cylinder 4-5 or 5-6
1/2 or 7/16 inch socket and one or two different length long extensions big fat reasonable quality socket for everything with easy access
Buy small 11/12/13mm crap from the middle of lidl/aldi thin wall socket and its extention and an adapter for you other long extension for undernearh cylinders between 3-4 or 4-5
a magnet/propelling magnetic pickup tool, for picking and placing steel D washers cheap chinese crap does the job
a tiny mirror on a stick as before spend $1, lets you see round corners. is that D washer on and in the right place....?
new remflex gasket
work out position of all nuts washers. note that down (manual or from the car)
use the combo of tools above to undo all nuts but then use the magnetic pickup tool to grab the d washer in order, if you undo nut, pick up the washer NOW, otherwise you will lose it....
this works for all d shaped washers
it won't work for the special round brass ones with the cone shaped hole. you will have to do them by hand and they have special pinch nuts with a split lead on them for the wedge shaped hole in the washer
put card or old dust sheet under car so you can just pull it out to retrieve what you drop.
if something you drop can't be found
1) on the pan rail
2) behind the bellhousing to engine support bars
3) on top of the k frame under the engine..
4) wedged between starter body and engine block..
5) bounced and went into the only open hole on the top of the k-frame never to be seen again

(ask on here for another)
Put new gasket on
mount up the manifold and hold it with the middle top fastener, or in my case on a different engine either end, but we are talking the brass washer and pinch nut positions
curse sweat jiggle it about so it sits right
curse swear as you try put put on the washers and nuts. finger tight
top= easy
bottom is not
magnet and screw driver to land the d washers on the studs at the bottom, do one at a time nut directly after washer.
followed by box spanner or cheap crap thin wall 1/2 in socket to place and start nuts
you may have to put 2 nuts in the box spanner to keep the first one at the end so it can be screwed onto the stud
when all just snug
check work and then go over all from the middle outwards doing them up properly remembering which combo of the motley set of tools above you used for each position
anything you can't get to from above. although you should be able to get to all, if you have the selection of tools above
can get attention with the ring or open end spanner from below... but that is admitting defeat....
do some more cursing and swearing
start it, listen for chuffing or ticking if any evident, switch it off, and find out why
if none idle it till its hot
check all nuts again
good to go
Rusty and Dan are on the button when they say 1) you can do it 2) be aware of the fact that a few places will take advantage if they think they can... they are not thinking customer for life anymore, they are thinking steak and beers friday night....
my method might not work for you, but its worth a try as its worked for me for 20 years,
mine has aftermarket inlet manifold and aftermarket headers on a hemi 6, everything is in the wrong place, the inlet runners are too tick to easily fit D washers and the flange on the headers is thinner than the inlet ...

for the lower back mountings there is a header pipe or carb in the way for all but the very end mounting stud, hence the use of THIN box wrench and Thin cheap socket on a very thin extension to get through a 1/4 inch gap between the manifold supports and the exhaust header pipe that goes around the starter. you may not have to deal with this crap...
start with standard tools and buy the suggested workaround tools if you need em
Dave