the cheapest most beefy thing you can do other then the obvious you mentoined above is to:
Stud the bottom end and have you some main cap Girdles machined..basically all the girdle is a 1 inch thick tool steel metal that is machined where it lies flush against your main caps..
the studs you will use will have to be the longer ones to go thru the main caps and the homemade girdle...this will strengthen your mains for an easy 700hp..
now keep in mind that when you stud a bottom end you will need a minimum of a line hone if not a line bore..depending on how distorted your mains are.....the studs have that much tensil strength over regualr bolts..
this is still much cheaper then going the other route...
also i have ran these type girdles on my last 3 small blocks..one made over 1,000 hp (blown)..
hope this helps..maybe someone else will have an idea for you also.
Brian
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Bjuice..
"I'M YOUR HUCKLEBERRY ! "