Mark,
Welcome...
As an alternative to trying to get 300HP from the 250... consider installing a GM late model inline 6 from a trail blazer. They are either 275 or 290 HP stock. For a stock engine run the GM PCM. Easiest method is to buy a rolled/crunched envoy/trailblazer. These engines are known here are the 4200. GM knows them as the LL8.
If you get lucky a complete wreck might be had for about $1500-2000.
There will be some fabrication to get it installed. Mainly engine mounts. An aftermarket rear sump oil pan is available.
If you have done a MS conversion then wiring to do the conversion most likely is doable.
