1977 chevy v10

I have a 1977 chevy c10 short wide pickup that I
just bought that sat for about 7 months. It cranks
up just fine and drove fine for the past week.
Engine still runs strong and has strong reverse.
Over the past couple days driving in a forward gear
has taken higher rpms gradually to accelerate until
today where it just won't accelerase at all buy
shifts into gear fine. I can lift the rear tires off the
ground and put it in any forward gear and the tires
will spin but very slowly and takes a bit to speed
up. I have yet to change fluids in it and figure it may
be that the transmission fluids/filter are gunked up
or something. I have checked the vacuum lines and
they are all good. Any information helps thank you.