I have an 86 305 from a G10 van in my 79 C10 pickup,(longbed too!)--I was actually pleasantly suprised at how peppy it is!--but compared to the 250 six it had before,I guess anything would feel faster!
Yesterday I drove it 130 miles round trip to mty sisters and picked up an engine crane,motor stand and a lawnmower her husband wanted me to fix(he gave me the crane and stand,a guy he works with was going to throw them in the dump!
)--despite heavy rain on the way home,and a headwind,I still squeezed 16 mpg out of it,going 65-75 mph!....
Yours might have low compression,or maybe weak spark if it seems sluggish under load--might check the coil or try another one(make sure the color codes on the wires match!--the coil,module,and pick up coil are a matched "team",from the factory a spot of white or yellow paint is on the module,and the wires on the pickup coil and ignition coil must match--if you swap in the "wrong" one,it will still run,but have much less spark voltage and that will reduce power)...Also as suggested your timing might be retarted,or possibly the centrifical advance weights under the rotor might be rusted in the retarted position--twisting the rotor,it should "snap back" when you let it go if the springs and weights are working right...
I recently switched my vacuum advance hose to manifold vacuum instead of ported vacuum--after yesterdays run,I think it did give better mileage,and the idle is MUCH smoother now too--I think I could give it a few degrees more advance without it pinging,I might try it--anything for more MPG!!--gas sucks at 2.28 a gallon!--but I went 130 miles on 8.3 gallons,not bad for lousy weather and loaded down some..but this 305 is the first chevy I've had that runs better on manifold vacuum rather than ported!....
In my truck I think it might have fairly low rear end gears,that always helps acelleration(I havent looked at mine to see what ratio it is yet!)---if you have 2.56 or 3.08 gears,it will be a dog with ANY motor,but especially with a small one like a 305...GM did that a lot in the 80's,a dinky motor and tall gears--great for cruising at 80 mph,but doggy on hills and take offs..
You also might want to check your engine codes and make sure it really IS a 305!--one friend complained his recently installed "305" was a dog--after I spent a whole afternoon checking things for him with no improvement,I decided to try looking up the code so I could look up the timing specs--it turned out to be a 267!!,from an 81 malibu!.(and timing it didn't help much!)..
