No, don't bore out your bellhousing.
If you want to keep your sm420 bellhousing that has the 4 11/16" bore, take your parts to a machine shop and get a custom plate made like I did.
I did not bore out my bellhousing. I took my bellhousing, and the whole nv4500 (after swapping the input shaft and front bearing retainer to the Dodge gas parts) to the machine shop, and told them I needed 1" of space between the transmission and the bellhousing (I brought them a 1.25" thick plate so there would be some metal to protrude into the bellhousing bore), and I needed one end of the plate to fit the 4 11/16" bellhousing and the other to fit the Dodge bearing retainer (I forgot what the diameter of the retainer was). They drilled and tapped the holes for the bellhousing as well as the transmission, and viola! perfect fit. [img]
http://67-72chevytrucks.com/vboard/attachment.php?attachmentid=1205117&stc=1&d=1390588516[/img][img]
http://67-72chevytrucks.com/vboard/attachment.php?attachmentid=1205118&stc=1&d=1390588521[/img]
Otherwise, the simpler route is to just find a cast iron sm465 bellhousing (already has a 5 1/8" bore that the advance adapters plate requires and will mount to your crossmember and motor without modification) and order the advance adapters plate.
Also, I used the GM nv4500 because I wanted the shortest length possible (the dodge versions tailshaft is super long). You don't have to change your motor mounts to use that trans. It just hangs off of the back of your adapter plate and bellhousing like an sm420 or sm465 or one of the old clarks. I just bought the Dodge gas input shaft and front bearing retainer which are one inch longer than the GM parts. I also had to get a different throw out bearing, but I used my old stock clutch and all my stock clutch linkage.
