if you don't want to swap in a junkyard engine.. and rather build off what you have for now.. and build what you want for it later..
here is what I'd do...
find a 305HO in a monty SS or t/a,firebird,camaro..
in the SS the eighth vin # will be a letter "G" .. the foby it might be an "H" have to check on that..
grab the heads and if you want to run a q jet carb the intake.. if you'd rather a holley/etc get a edle intake used..
the 305"g" engine heads are the best flowing 305 heads.. and the chamber side is tiny iirc 55 cc.. that 'll get the compression up to about 8.9/9.1
in that 307.. add in a smallish cam (think 305 truck towing cam)
and it'll wake that 307 up..
best part .the parts are dirt cheap..
you can get 300hp out of this set up...
good luck... the swaping in a junkyard 350 isn't a bad one.. but food for thought..
g.m. sells the 96-02 truck vorect 350 all new.. for 1999.oo and with 1200.oo budget,, save a little more and you are there..
depending on where your at.. pace.com or s.d will have the new engine on the cheap
add intake and carb and headers and you have a stroung new 325-350hp 350 small block thats all new parts... with a roller cam...
if I keep a small block in my truck (71 c-10) this is the engine I'll install.. you can't build one for what g.m. sells it for..
I have a 496(489) in basement that I'm fighting with do I, or don't I..
good luck.. with whatever you choose..
oh and welcome