305 heads on a 350 is an old low buck circle track mod. Typically on budget enduro's and "farm built" street stocks. I've never been impressed with the swap and these days low buck tricks won't win a race.
The 305 heads don't flow enough for decent performance. The compression ratio increase won't offset the low flow. It's only about 175 cfm on the intake side.
You'll get about 25 hp on a typical 8:1 350 from the CR increase which will bump to about 9.7.
That's like 245 hp from 220. Not something to brag about.
You could professionally port them and get an additional 35 cfm at .400" which only gets you to bone stock Camel hump flow of 195-203 CFM.
If you think it's cheap and easy, Brzenzinski charges $2195 for modified 305 heads.
You'd be better off with changing the pistons to get 10:1 and using better flowing heads.
Besides, at least one of those 305 heads is very likely to be cracked.