HELLO,
EASY SWAP. FIND/USE THE V8 SIDE MOUNTS AND REMOVE THE CROSSMEMBER THAT YOUR BELLHOUSING MOUNTS TO. INSTALL THE CORRECT CROSSMEMBER FOR THE AUTO TRANS, POWERGLIDE, TH350 OR TH400 WILL BE FINE OUT OF ANY 60-72 CHEV PICKUP PREFERABLY 1/2 TON. THE ONLY ISSUE NOW IS THE TRANSFERCASE. NEED TO DECIDE ON DIVORCED OR MARRIED, AND THEN THE DRIVE LINES TO MATCH.
AS FAR AS THE 4.3 V6, IT WON'T MAKE ANY DIFFERENCE AS FAR AS IF IT WILL HANDLE THE HIGHER RPMS BETTER THAT THE SMALL BLOCK. THEY SHARE THE SAME STROKE COMBO JUST 2 LESS CYLINDERS.
FOR THE 5 SPEED USE THE TRANS/TRANSFERCASE COMBO OUT OF A EARLY TO MID NINETIES CHEV/GMC. USE THE CROSSMEMBER FROM A 73-87 1/2 TON, YOU MIGHT HAVE TO REDRILL THE HOLES IN THE FRAME. ANYTIME YOU DO THIS TYPE OF MODIFICATION FROM STOCK YOU'LL HAVE FABRICATION TO DO.
GOOD LUCK MAN!
HOPE THIS HELPS
BRANT.