Hi there Just10Time,
I've been an IMCA modified builder and driver for several years in southern NM and west Texas. I have an 05 KraftKar 4 link on Chevelle chassis. Willie Kraft sold to Derrick Ramirez Motorsports a couple of years ago, and I believe Ramirez is currently holding top honors in the manufacturers challenge right up there with Larry Shaw and Jeff Taylor. Derrick is kicking their butts in points standings right in there with Jeremy Payne and Jason Hughes.
Anyway... for several years a 383 stroker was the winning ticket... (3.75 stroke with 4.030 bore and 6" rods). Using a seasoned GM 350 block though left much to be desired in the durablilty and longevity area.
But, now that IMCA has a rule change allowing aftermarket blocks due to cost efficiency, and Dart SHP has made buying an aftermarket block so much more affordable and no brainer over using a junkyard block, everyone is heading towards going as big as they can get for bottom end torque and taller gears.
I just got my new Dart [Little M with 400 mains] back from my engine builder, punched out to 404 ci (4.125+.020 due to rebore/rebuild).
My engine was originally a 388 stroker built with 4.125 bore, 3.625 stroke, 6" rods, and (illegal for IMCA) big roller cam. It was a screamer, but mistakes cost me several burnt pistons so my builder convinced me to bore and stroke to 404 and make is IMCA legal.
If you follow other threads here on RJ, there is much opinion about staying away from 400 mains if possible, which is probably one good reason why Dart only makes the SHP block in 350 mains.
With an SHP block, Dart only recommends a max of 4.165 bore and 3.75 stroke, to give a 409 ci. There is much debate about using 6" rods over 5.7" rods, so that is up to your choice of pistons you want to use.
Some guys are experimenting going bigger to 414 and 420 cubes, but that is really pushing and streching it in machine work, and not leaving much room for a future rebuild without special order pistons. It appears the big bore and long stroke is heading the right direction for dirt tracks.
I highly recommend a new SHP Dart block, finished at stock 4.125 bore, and 3.75 stroke, which is of course a 400 ci platform. This will give you two more standard overbore rebuilds in the future, knowing how hard dirt racing is on engine life. The big bore and long stroke will give you lots of bottom end pulling torque allowing you to use a taller gear to prevent blistering the tires coming out of the corners, yet a lot of speed at end of the straights with lower rpm range under 7800 revs.
Good luck, and let us know what you come up with...
Jim