The perfect answerYou need to qualify or expand what you mean by "good"
305's are dependable motors , and will respond within reason to the usual
performance upgrades such as a good intake manifold, headers/exhaust and cam changes. The heads are a choke point and will limit what you can do with simple bolt ons. The 305 HO heads being the best of the stock 305 heads.
You can put on heads with 1.94" intake/1.5" exhaust valves but typically 350 heads have a 64 or larger cc chamber and thats too much as the 305 heads are around 55 cc so you have to mill the heads to get the c/r back up.
People have reported good results with milled 350 vortec heads on them.
If you have a tight budget and have a good 305 on hand theres no reason to not use it as most bolt on performance parts can be later moved to a larger sbc when your budget permits.